Even if it is a brackish fish, i believe you can slowly ween them to be 99% freshwater fish
They won't live as long though, in full freshwater, if they are a brackish fish. Yes you can put any fish into water that isn't suited for it and it will live for awhile, but not like it would in the appropriate water. If the fish is brackish, keep it in a brackish tank with other fish that like that water.
The planet catfish log on the Synodontis angelicus says it gets around 9.5in. Even that is too big for a 25 or 35g tank. I'd take it back and get some cories that are small enough to live in your tank. Or get like a 100g tank so you can have a group of those guys.