My favorite set-up is the Dwarf cichlids.
There are lots to choose from.
You can add other fish with them.
Most are easy to breed and very interesting to watch with their young as they escort them round the aquarium from pit to pit in search of food.
If you go with a 75gl tank you can set up a harem with 1 male as 3 to 4 females and watch the male wander from female to female but if you do that make sure that the females have plenty of caves, flower pots coconut shells and bog wood and the like with tall plants to define the Territory's.
my favorite Dwarf cichlids are
Apistogramma borelli
Apistogramma cacatuoides
Apistogramma hongsloi
Apistogramma inconspicua
Pelvicachromis pulcher (krib)
all these can be kept with Tetras and small Barbs Like
Neon tetras, Cardinal tetras, Glass catfish, Tiger barbs and the like.
I would have 1 x Male Dwarf cichlid, 3 x Female Dwarf cichlid, 10 x Tetras, 10 x Small Barbs, 6 x rasboras, 6 x Corys and 10 x Hatchet fish.