For catfish I say a medium sized pim or large driftwood cat or a bagrid, examples are sun cats, ornate pims, 4-line pims, Asian upside down cat, trachy trachy etc. as for an eel, only eel like fish you'll be able to keep with the rest of the stock your aiming for would be bichirs, other eel like fishes will be to thin and snacked on or to large for the tank. And as for predators, you can look into characins such as wolf fish, payara (excluding armatus), cudas, odoes, certain "pike" species, or you can maybe try some gobies such as what's typically sold as a watercow (eleotris picta) or a marble goby (Oxyeleotris marmorata, though this with good genetics may outgrow the tank or take it all to itself depending in the specimens attitude, same with some other fish mentioned). You can also try oddball catfish for an odd predator, if you can find a fowleri that'd be a nice one.
All of these different combinations will always have a risk, some more than others, but for the most part you can keep most of them together, when you find specific fish you are looking for, tell us and we can share our knowledge on how they do together. A lot of it will come down to the specimens you get, some may be to aggressive some may be to much of a push over.
A gulper you'll probably grow bored of as they are slow growers and aren't to active and you won't be able to keep much with it.