A lot of pike cichlids will work in that, assuming it stays around 10 inches or smaller. I can't think of any appropriate tankmates you could hold in there with him.
Im a big fan of crenicichla regani both orinoco and tapajos variant is gorgeous. If you decide to go with tapajos you can put them with some lemontetras in a plantedtank and you have a small piece of biotope from rio tapajos