Endgame319...
Your possibilities are almost endless. One thing you need to consider is that Serpae Tetras, Hyphessobrycon callistus, have kind of a Jeckyll and Hyde reputation. They can be perfectly peaceful community fish but are also known as fin-nippers, especially with long-finned fish like fancy guppies. A big school of Serpae Tets, on the other hand, would look incredible in a planted (live or fake) tank with other fish that can take care of themselves like other mid-sized Tetras. The Bleeding Heart Tetra, Hyphessobrycon erythrostigma, comes to mind. Both of them (Serpaes and Bleeding Hearts) like to be kept in schools of 8 or more.
Toss in a few durable bottom-dwellers like Corydoras or one of the smaller Plecostomus-type cats and you could have an absolutely gorgeous tank!