Well, Gymnogeophagus gymnogenys species are harem breeders and social fish, so they are probably happier in a group. They like finer substrates than what you have in your tank. The JD and GT will most likely grow too aggressive for your Gymnogeophagus. They may also prefer different water parameters, although the water in Uruguay is not as soft and acidic as in the Amazon....
Not saying it's the end of the world, but in general we should know more about the fish before buying them. Very often LFS staff are not exactly trustworthy...