Sounds awesome, glad you know how to take care of the problem with the small tank
However, buying a fourth fish for it wasn't the best thing to do at the moment. But, as long as it goes well and they don't seem distressed.
Your new tank is roughly 60 gallons, which would fit just about four-five comets, note that they want and need more space than fancy egg-shaped varieties.
I would stay away from the sand, really! It's not a suitable substrate for goldfish as they enjoy picking the gravel up in their mouths and spit it out. Sand would most likely be swallowed real easy and the goldfish would start getting transparent/sandy poo, not good. So if you can, and have gravel in the small tank, why not buy a few extra bags of some pea-sized gravel and mix it with what you already have? Condsidering that you're moving the fish while the tank cycles. Note that you need to let it stand empty for at least one-two days before adding the fish! (Temperature reasons)
I'd stay away from the bristlenose. I have a really huge pleco living with my goldies, and the reason why it works is because he's actually taught them where his food is and that they shouldn't get close when he's eating
In the start they ate all his food so I got really worried :/ Bristlenoses grow so small in the end, and they are very easily startled when a goldfish comes to nibble their food! Besides, you really don't need anyone to eat algae, as long as the tank isn't over stocked and you don't keep the light on for too long!