Most cold water fish won't grow as big as goldfish, and are still better off in a pond since many of them are shoaling fish so keeping just one of them alone with two goldfish probably wouldn't be the best idea.

Some people swear against plecos as well for the fear that they'd eat the goldfishes' slime coat, but so far I've had my pleco with them for a year and he's been a very kind-hearted one c: (Except when they come to his feeding spot, he gets mad, but the goldfish know exactly what distance to keep nowadays!) Personally, I think they are a really great help! Getting rod of any left-over foods that the goldfish wouldn't find or scraping the algae off of the driftwood and plants (Mine got lazy though, he's only on the driftwood lately...) If you like plecos, you could get one! But make sure that you know what kind it is, how big it would grow, and that it enjoys similar water quality as goldfish do (Temperatures between 22-26 degrees, and they'll thrive together!) Another species of suckermouth catfish I've had success with are the ancistrus, or Bristle noses as they're commonly called. They grow to be roughly 8cm long so you could keep 2-3 of them instead of one goldfish I imagine, since they're not messy at all

Apple snails are sexed and then breed (I think they lay eggs??) as opposed to most smaller snails that basically just clone themselves D: I have 5 of them in the main tank, and I first had one with the Platies but they seemed to be nipping on the snail's shell all the time so the poor thing was afraid to move. He's a lot happier living with Danny (the pleco) and the goldfish They don't take notice of the snails anymore, but sometimes Danny is slapping them around with his tail not noticing... x'D
So if you'd want to go goldfish all the way and skip the plecos, you could add a few more snails, maybe 3 would be enough in that tank? Apple snails also eat algae and help as much as they can keeping things clean (In nature they can grow to be 20 cm across the shell).
There are people however, who haven't been as lucky with their snails and the goldfish were constantly trying to eat them resulting in the snails' deaths... D:

Why not put them in your parents' pond? I bet the snails would be happy there! And I think they wouldn't overpopulate the pond because of the fish and whatnot that live there anyway, they might even eat the snails or their eggs, if they're that small.

Yeah my dad mentioned his ancistrus cat fish may still be alive in his tank! (Theres alot of algae in there!).
But he was talking about finding it a new home down in Devon as it's been living in a tropical tank all these years, must be very old by now if it is still in there! (Not to mention I think the journey here and then leaving it in a cold water tank with a goldfish, would be very stressful!). But I do love the little guys!
But I also love snails too

So either 2 goldfish and a catfish, or 3 goldfish and a couple of snails =)
I might go for the snail option, add in one or two and see how they get on with the goldfish.

Yeah, I think I'll let my mum take the rescued snails home (I think their common pond snails so can grow up to an inch!) their ponds don't have any fish, but lots of frogs and newts! So I imagine they'll keep their population down.
As I suspect keeping them in the tub and eventually they'll just multiply and take over! Unless I remove the eggs? I read their easy to control, but maybe a nice big pond would be better for them!

Our ancistrus are pretty old by now too, I think they should be closing in on 5 years? They also breed regularly so my sister brings the babies to the pet shop every once in a while
I doubt he'd be stressed living with goldfish since a good temperature for goldfish is 25 degrees up to even 28. So even if they're called "coldwater" fish, they don't live in cold water Unless you have them in a pond of course!

That's of course a possibility, he'd probably be safer and happier moving into a cycled tank right away

It's so sweet that you're taking such good care of the snails :'D If you feel like it, I bet they'd be happy to eat a crushed flake every once in a while too

We had ones similar size but with strong red color... they were allover the place, inside the filter and outside the filter... D: In the end we really didn't bother looking out for them while vacuuming the gravel. We got rid of them that way, and also, I think mom was so sick of them that she simply crushed whichever she saw whenever she sat them D: Nice snack for the fish that way though.

They are a bit of pests in the end, but they do look beautiful when they live with guppies and some fine-leaved plants I had them that way in my room, it was just the main tank they were a problem in

What about shrimps? Ghost shrimps can live in a variety of temperature ranges, eat leftover food, clean the tank, and if it's really hungry, it will go to the surface to eat! There are translucent white but there are over more colorful shrimp that basically do the same thing. Maybe because I'm more of a tropical fish person (I don't have space for goldfish), but I think shrimp would be a good idea.

Nossie: the snails loved the fish flake I put one crumbled in yesterday and a lot of it's gone now

I think I'll stick to snails, they seem to be the simplest of all potential goldfish companions I'll introduce one or two snails, once I have the bigger tank with the 2 goldfish in and see how they all get along.

I was considering weather loach, but for some of the breeds you need a licence and to keep them in 2's or 3's, not to mention they can grow pretty big, not sure 2 would really thrive in a 125 litre tank.
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I did also come across a hillside loach, that sort of looks like a pleco that would be nice to have, but they like strong currents, so again, not sure my tank would be the best conditions for one.
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I know! But after researching, it says they prefer tanks with lots and lots of current, so won't thrive in a calm tank like goldfish do.

Yeah I thought as much, so after some research decided it would be best to leave it at snails!

I did see a pretty awesome looking hermit crab in the aquarium shop the other day! I imagine with crabs they ideally need some land as well as water, so again, probably a terrible mix with goldfish! Not to mention it'd probably try to eat Lucky XD

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They are so cute though!

I just think hermit crabs look awesome... Shrimp can be cool as well, but I don't know too much about shrimp... Well apart from the Mantis Shrimp I researched about for a while, those guys are just incredible creatures.

I'd love a mantis shrimp too one day! This little guy is the Peacock Mantis Shrimp ^_^ Beautiful things....

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Also an update with the snails, I've lost sight of their eggs and there are no baby snails.. I highly doubt the eggs would have hatched after a few days as well... Bit of a mystery that... It's like the eggs have disappeared and I'm pretty sure the snails wouldn't have eaten them?

I saw them yesterday resting on the bottom of the tub (they'd fallen off the plant at some point) and I was very careful to clean around the eggs and add in some extra water, only now they aren't there? Strange...

I also read that snails can carry diseases? So maybe not the best idea to un leash them on the pond at home?

Don't ramshorn snails have a curly shell? Like this:

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Mine definitely don't have shells like that... I think there was one Ramshorn snail in the plant, but I thought the shell was empty and didn't put it in some water, so that snail didn't make it...

The bladder/pond snail looks more like my little guys:

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Well so far the 3 pond snails I have in a little bucket are fine, they did lay some eggs, but then the eggs disappeared after 5 days
No baby snails in sight, so no idea what happened there!

I'm considering keeping them for now and then perhaps in the future having them in the 21 litre tank with a betta or some guppies (if I add a heater) 

Sorry I thought the curly shelled snail was yourse.

I have danios in with my goldies but one day I fear they may fit in goldfishes mouths