The RC cars that come on either 27 or 49 mhz are on a different frequency than any hobby 27mhz car so no worries there...

As for the twitching, are you stuffing the antenna inside the balloon with the receiver? Because it should hang out and go through the antenna tube like normal.... it that's not the case, try replacing the crystals.... I had a duratrax receiver that a traxxas controller one channel away would do that too (servos would twitch, and steering seemed to affect the throttle) Could just be a bad crystal or crappy radio (redcat=crappy)