I need to use a small magnet to hold a BB in place in my new cannon mod. Will having a magnet near R/C equipment be a problem? It would be very small, say 1/8" long by 2mm wide, just enough to hold a bb in place while the tank jostles around looking for a target.

try and see i guess. if it dose, try a bigger antennae

No should not be a problem ..you have magnets in the gun motors and turret unit plus the drive motors...should be ok.

Should not be a problem. Only alternating or moving magnetic fields like motor rotors make radio interference.