Hmm, well, personally I'd avoid the use of "powers" in a Serenity system- feats, combat abilities, skills, and maybe some powers that can be represented as training. Heck, I remember at least one build in the core book that didn't have a single power, so it can obviously work. And ~PL 5 sounds good.
Now, I understand there were some "psychic" stuff on the show- but that really seems like it should be the realm of the GM. After all, River seemed more like an NPC than PC to me.
Finally, later in the game, devices- Ancient artifacts, personal shield, zat guns, etc.- would come into play.