Your car servos will work just fine. Servos are not frequency or band specific in any way. Nor do servos know about FM, AM, PCM or whatever. They all work on the same way when it comes to RX->Servo communication.

If anything, car servos are often stronger and faster than needed for your average trainer or sport plane, since the stearing linkage on some cars takes a lot more abuse than you'll dish out on the controls of an airplane in flight. ("in crash" is a different story, but that's another story).

For flying glow powered planes, there are lots and lots of options for radios. Unless you have a Ham license, you'll need to be on 72Mhz in the US. Most radios are FM or have FM and PCM ability. FM is fine. There are some AM sets around. Some of them are fine, some are low-power sets intended for small electrics. To be on the safe side, go wtih an FM Transmitter. And, to be on the safe side, look for a Receiver that is "duel conversion", or is a JR brand RX intended for airplanes. (JR RX's aren't duel conversion, but they have their own custom way of doing things that is as good as any duel conversion RX out there, so it's the "exception".