RE: Is this a dumb idea??
If cost is an issue and you don't care about doing hardcore 3d then the parts you have will help to feed your itch to fly a rex. I've got a cheap-rex (SA w/ hs-55 servos and mediocre align gyro) that I keep around as a spare copter and building an Airwolf body for it. For just simple flying around it's still pretty good. But I can promise that you're going to want to upgrade as soon as your skills progress. I haven't tied a battery as small as yours in it, so watch the cg since it may make it tail heavy.