I'm sorry but I wouldn't suggest anything Hydrocynus, Brycon or Hydrolycus for cohabitation with a Gar. All of them generally prefer faster flowing waters than Gars and all of them are fairly darty swimmers and aggressive feeders. Being fast moving fish they can potentially spook the Gar as they grow larger causing the Gar to jump or faceplant into a tank wall. At smaller sizes that very same movement will attract the Gar as potential prey.
That's just my opinion but it comes from personal experience with several different individuals of each of the species at various sizes. Even now I'm having an issue making sure my Panda/Pied Florida gets enough food in his current cohab. I have to hand or tong feed him as otherwise he gets outcompeted and does get spooked at feeding time because his tankmates are around his length but much girthier and much more aggressive feeders.
