yes. adding three different types is just like having 3 normal ones. its all up to you. but id add the silver aro first, and wait til it gets to a good size to where it wouldnt get eaten by the knifefish, clown knifefish has deceivingly large mouths. and then once you get the clown knifefish and the silver aro in the tank, be a good judge on wether you have room for a few oscars, and mind water quality with the oscars. they are messy