Hard to tell what species from these pictures but it's definitely a sucker (loose term to define Catostomids). I'll gamble it's a Northern Hogsucker though from that coloration and the bars. For NHSs, keep them over a mature substrate they can sift (sand or loose gravel). For first foods to with live earthworms and then slowly wean them on to "melty" shrimp pellets. Melty meaning they're pre soaked in water and the soft texture of them at that point will make them more appetizing. For my white sucker, I started him on bloodworms and in about a week he was eating large flakes from the surface, but that's very unusual behavior for a sucker. No worries as far as aggression, pretty much all of our native Catostomids are peaceful fish that mind their own business. I'd see no problem keeping a 12" sucker with juvenile 1" darters as suckers just aren't morphologically built for predation, what with their inferior mouth position.