If you're talking about a pleco those can grow up to 3 ft .  You would need a huge over 100 gallons to house a normal pleco.  Also, plecos are tropical, goldfish are coldwater.  They require different water quality and temperature.  They sell plecos and goldfish when they are small not adults yet.  Your goldfish alone may reach 1 foot!  If you get a 10 gallon tank for your goldfish, then there won't be room for any other fish in with him.
The nitrogen cycle is fish wastes, that turn into ammonia that then with beneficial bacteria eats the ammonia and then that turns into nitrites, more beneficial bacteria eat the nitrites and eventually when your tank is cycled you will have zero ammonia, zero nitrites and 40 ppm or less of nitrates.  You will need a master test kit to test how your tank cycling is coming along.  A fishless cycling process needs to be jumpstarted with beneficial bactera.  You can also place a bit of fish food or your dirty fish water in the tank to help this cycle get going. 
Ammonia and nitrites present in the tank water are harmful to your fish.  They can die as these are poisoness commponents.  This is what is called "New Tank Syndrome".  Fish can tolerate nitrates but not in high doses that is why you need to test your tank regularly.  It takes about a month- 2 months for a new tank to complete it's cycling.

Im pretty sure I read somewhere Pleco's needed a different temp than Goldfish, I mean, I never see them in with regular goldfish.

If you'll be getting a common goldfish (or comet or shubunkin), they need pretty much of space to swim, and a 10 gallon IS small... so if you definitely want the 10 gallon, keep something fancier and smaller in that, like a common fantail, a black moor, pom pom goldfish... rather not an oranda, because these fishies can grow up to 12 inches.
So if you intend to use this tank for a common/comet/shubunkin/oranda, get yourself a 20 gallon if you can afford it instead of upgrading over and over.

And Mindemae pretty much covered the cycling process But you could speed it up by using water from your second tank (if you have one) and fill it about halfway or less with that. Or by "filter media transplant" Where you use filter media from th current filter and add it to your new one. But it's sometimes better to just start from scratch, and let "nature" have its course

Good morning Wandafish.

Well thats fine. a ten gallon will do you for the time being. and even so, you can use it as a hospital tank later on

Yeah, I have a 10+ gallon right now, and... to be honest, it's a huge pain to look after it. I'm so happy that I will soon have my 83 gallon up and running! xD For now it's standing upstairs, waiting for a place to be... so, it is alright to keep a 10 gallon with a goldfish in, but then be prepared on some extra work!

Although, if you only want one goldfish, I could recommend you getting a biOrb, they're sold in sizes of 8 and 16 gallons, and it's like a walk in the park (without a dog) to clean it and take care of it as long as you keep only one goldfish in it I was using it as a quarantine for now, and it's working perfectly as long as you clean the gravel well during the water changes, and as long as you stick to floating food Check it out, it's quite cheap actually! But be warned, ONE goldfish only if you want to keep it long term!