Filamentosa Barbs (Not aggressive, but highly active, show fish.) 1-2M + 2-4F pr
I have Senegal Bichers. There are others, more colorful. Predatory. Will eat what fits, and they can open their mouths big enough to eat some 3" long fish. Not aggressive.
Snakeskin Gouramis- One of the few that are OK kept several per tank. Try 1M + 1-2F, and have some upper level plants, driftwood or something.
Knives- Generally peaceful, but make sure they are big enough not to get eaten. They are slow growing, and the Bichers are fast growing. If they start out the same size the Bichers might get big enough to eat the slower growing knives. Look into some of the glass knives, African Brown Knife, and others that won't outgrow the tank.
'Eels', (Mastacembelus, several sp) Check the maximum size. They are social among themselves, probably predatory toward small fish.
Mid-sized Cichlids from South and Central America: Keyhole, Firemouth, Hero, Severum... obviously a lot more research into species, social needs and so on. Generally most Cichlids can be quite aggressive defending their territory, especially when they are paired, M+F and are in the mood to breed. Some are relatively peaceful toward other species, as long as the others stay out of their space. Make sure to research breeding behavior, though.