Funny you posted this, I just purchased 4 more of these little guys today from my LFS. I have had 3 for about 8 months now and love them. I will do what I can to answer your questions.
They do not get big at all. Planet Catfish says between 5-6" is the max, but that depends upon the specific species of banjo that you get - I believe that there are a few different species on the market. The ones that I have had for the better part of a year have not grown significantly- from what I can tell. So, not only do they not grow big, they don't seem to grow fast.
I would recommend getting them in groups of at least 3. Although I would not say that they are a schooling fish, mine tend to hang out together. And since they can be pretty elusive (and inactive) fish, having more of 'em dramatically increases your chances of seeing them. For a 75, depending upon your current stocking situation, I would recommend up to a half-dozen. Shouldn't be a problem because they are pretty affordable little guys (I paid $2.50 per for my group today).
In terms of special requirements - just some hiding places. Since I relocated my original group from an all-gravel aquarium to one that has sand in a corner, I see them much less. They love burying themselves in the sand during the daytime. Sometimes I see one's tail sticking up out of the sand, lol.
For their ideal diet, I am not sure. Mine seem to be doing well and, for the most part, I put shrimp pellets, algae disks, tubifex worms, and the occasion bloodworms in the tank. I can honestly say that I have NEVER seen my banjos eat, not once. But they still live, so they must be finding something out of that group to eat. As for keeping the bottom of the tank clean, I cannot give you a good answer on that since i have never seen them actually eat. But my tank floors tend to stay pretty clean, so they may have something to do with that. As bottom-dwellers they undoubtedly eat mostly off of the floor.
Sorry, but I cannot help you out with the fish compatibility question, as I have no idea what this fish are. But I can tell you that the banjos, while docile, are a very hardy fish. Wish I could help you out there more. The banjos may be so inactive (at least during the day) that your fish may not bother them.
As for nocturnal, absolutely. I do see mine during the day sometimes, but that is usually because they found a hiding place that I could see into. If you want to see them in action, I highly recommend investing a a nocturnal "lunar" light. That was one of the best things that I bought fish equipment-wise.
Hope this helps!
so im trying to figure out what to put in the tank with them to help keep the bottom clean)
