There are hundreds of species that fit your requirements but the trouble is finding them in the fish stores.
Some reasonably common ones to look out for are
Morgurnda morgurnda or Morgurnda adspersa, purple striped or purple spotted gudgeon (4")
Chaca chaca, frog mouth catfish (7")
Channa bleheri, rainbow snakehead (8")
Chana orientallis/gachua, ceylonese green snakehead (8")
Dwarf pike cichlids like Crenicichla reganni or Crenicichla notophthalmus 4 to 6 inch)
Monocirrhus polyacanthus, south american leaf fish (4")
Plus there are many more less commonly seen ones like rainbow wolf fish, butis butis, monster fish, and so on.
Some of them are single specimin only fish while others can be mixed with others of their own kind in species tanks or if you dont mind a bit of carnage and trial and error you can make aggressive communities up, just remember that the smallest fish must be too large to fit into the largest mouth and that if a fish has big teeth it will probably use them
