there is a book by Interpet called community fish. which lists what u can and cant keep together. main concerns to watch out for that i have personally witnessed , are
shcoaling fish , like danios, and minnows etc , must be kept with at least 5 other wise they will harass other fish looking for them to play , and things like angels that are slow will get stressed by an energetic danio wanting to play,
fin niipers tiger barbs cannot be placed with angels or bettas unless in a large tank and enough tigers to keep themselves amused.
territorial fish , Red tails sharks , ruby sharks , only one or two per tank , as long as you have hiding spots.
watch out for shop owners selling u 6 ruby sharks just cause they look good together , mine all killed each other because of territory ( how on earth they kept them alive in store i do not know )
my last line up was guppy, mollys , plattys , zebra danios , pearl danios , mountain minnows, 2 angels , a plec and a silver shark , harlequins ,
(silver shark and angels were not at the same time )
i had the joy of having a pair of pengasius cats for a while but unfortunately they did after 2 years , and someone putting a shell in my tank and not telling me .