Common Plecos are a member of the suckermouth catfish family. Here's what it says in -You and Your Aquarium- by Dick Mills:
"This very lage species has elongated, spotted, brown body, a broad, flattened head, and of course, the distinctive ventral sucking mouth. Formerly known as ~Plecostomus plecostomus~, it does a good job of keeping down algae growth in the aquarium. However, if it isn't given enough green foods it may nibble at your soft-leaved aquarium plants
Size: 9 3/4 ins long
Orgin: S. America
Sexing: No visible differences
Food: Cegetable matter
Water: All types
Temperature: 75oF"
From personal experience, Plecos are great fish, for tanks 30 gallons or larger. Many species do grown ten to twelve inches, and would need much more space. They can be agressive, staking out their hang-out and chasing other fish away. I've witnessed a four inch pleco chase ten inch Pacu away from algea waffers, nobody, not even Oscars, usually dare mess with a large one. They're also primarily nocturnal, so give them a hidy-hole during the day. Sublimenting the diet with cucumber or zuccini rounds and romane lettuce makes them happy.
Your pleco really ought not to be with goldfish, but if he's still living, damn, you're pretty lucky. Sometimes it works, customers do come in once and awhile and say they've got a pleco in their goldfish tanks, but I don't reccommend or sell plecos for that purpose. Plecos are tropical -warm- water fish, and goldfish are -cold- water fish. But hey, since you've got no where else to put him except bring him back to the LFS, might as well take care of him the best he'll do.
As far as other fish to go with goldfish...err...I don't reccomend any at all. Even danios or white clouds. Goldfish, espeically commets and koi, might decide to swallow 'em. If you want to keep a cold-water tank of just white clouds or danios and other cold-water species, then do so. But goldfish really ought to be a tank unto themselves.
~~Colesea