I have a 33 gal tank. I have all types of goldfish. The biggest if about 6 cm.
The slim type goldfihs like comets and common goldfish will definately outgrow th 33gal tank, the fancys (depending on the exact numbers of them you have), may overstock the tank as they grow- the pleco, depending on its type, could also easily outgrow the tank too. So you should be careful and wary of your fish and take into consideration that at least some of them may need to be rehomed or you will need to find a larger tank for them (a pond of 3ft depth or more will surfice for goldfish like commons and comets- such fish need at least a 75gal tank if kept in an aquarium permenently).
You don't need a heater in the tank depending on the type of plec you have and the general temp of the tank at room temperature- slim type goldfish like comets and commons are true coldwater fish and temps above 18 degree's are not ideal for them, while the majority of fancy goldfish are coldwater going on sub-tropical and temps around 16-23 degree's are fine for them (but they are adaptable fish and in the short term such temps won't do any harm, just make them more active and poop more)
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I also have to warn you that slim type goldfish can sometimes bully fancy goldfish, more so as they mature, so you should keep an eye out for any such behavior in your goldfish and separate any bully or victum fish should this happen imediately (fancys tend to be the victums in such situations while slim type goldfish tend to be the bullys).
What are the dimensions of the tank (length, width, height)? What are the exact numbers and types of goldfish you have in it at the moment? What sort of substrate do you have in the tank (i.e. like rough gravel, rounded gravel, fine gravel, rough/large sand, fine sand, pebbles etc)? What temp is the tank currently?
Depending on the tanks dimensions, stocking and substrate and temperature, you could have a weather loach in there.