What species are your catfish? They all have different dietary needs- but I have never heard that they include turtle pellets or goldfish foods. There are special catfish tablets that sink, and depending on the species, these can be supplemented by either vegetables (cucumber, courgette etc) or bloodworms. Some catfish also need to eat wood. It is important to get this right as your fish's health could suffer if fed the
wrong food.
I would have thought the convict and the jewel would be better off with tropical flakes, supplemented occasional live food.
As for the marbled goby, all the information I can find states that it is a fish predator. So why are you keeping it in a community tank? It may still be possible to train it to take other foods, but it will probably take some time. Ask for suggestions in the oddballs forum. Goldfish feeders are a bit risky as they are known to often carry disease. The safest way would probably be to breed your own feeder guppies- if you really cannot get it to take other food.
edit: rereading your post, I see that the goby is actually taking dry food. So all you need to do is to take it out of the community tank. You can't really expect it to go against its hunting instincts, or to understand that only certain fish in the tank are "feeders" and others are your prized pets.