What are the Flux battery compartment dimensions? I have no trouble fitting a 8200 mAh LiPo in the Revo battery compartment. It is a softcase, and it's pretty snug, but it works great.
The GensAce 3s batts should work well, as well as the Turnigy Nanotechs. But I don't own a Flux, so I can't say for sure. I just know what has worked in the Revo, which has a similar size compartment, afaik.