OK, I was away so didn't see this thread at first .... Well I think I can answer the OP's original question.
1. Goldfish are sub-tropical rather than cold water, so they are more a "cool" water fish ... <pun intended ... sorry!>
2. Fancies tend to prefer warmer water than commons - commons generally do best in high 60's, fancies in low to mid 70's. Fancies tend also to be smaller than commons and swim much slower, they are much more susceptible to predation/injury by other fish.
3. Small catfish (otos, cories) can be eaten by big goldfish. There had been cases of those catfish lodging in the mouth of the goldies (they generally have spines that are designed to prevent being swallowed). This usually results in the death of both fish. For this reason (plus others), the small catfish are not compatible with goldies.
4. Big catfish (commons, etc) can & do eat goldfish, esp. the fancies, which are not fast enough to escape. Even juveniles of the big catfish are not safe, as they sometime will suck off the slime coats of the fancies while they are sleeping, even when they are not big enough to eat the goldfish whole.
4. Really rambunctious cats (like pictus) will disturb the slow swimming fancies ... even if they were temp & size compatible (which the pictus is not.)
5. That leaves the dwarf catfish - bristlenose & rubberlip. These are not too big (4" or so) nor too small, and had been kept successfully with goldfish by many. As long as you heat your tank to at least mid 70's the cats should do fine. There is a small chance the dwarf may injure a fancy by sucking on the slime coat, but this generally happens only when the cat is starving. With enough algae (or proper feeding of the cat with veggies) the risk is minimal.
Finally, someone mentioned the hillstream loach. Hillies are not catfish, nor are they bottom feeders, or algae cleaner. They are cool fish, well suited to the river tank mentioned a few pages back .... but they don't have much to do with the original question ....
