i have severum, jack dempceys, redspot pleco, festivem, convict, jag, red parrot, blue acara and a syno cat in my tank with my oscars.
i started off with all of my fish from babys, this way they have all grown up together and get along fine apart from the odd flearing season between the smaller guys
most of the time i find that as long as the other fish wont fit in the o's mouth it will be fine, although sometimes they think that there mouth is bigger than what it is so make sure that it carnt bit half of the new fish.
i have a mate who also has lots of different types of cichilds in wtih his o's and some of them would easly fit in the o's mouth but they seem fine.
everys o's different so its really just trail and error but whats in my tank should be fine if theyre right size imo