May be I'm old school, but I still prefer to keep at least 8-12" between the gasser ignition and the RX circuits. That means EVERYTHING: switches, batts, servos, etc. This rules out mounting throttle servos near the engine. I know plenty of RCers get away with doing it that way, but I'd rather not tempt fate if I don't have to. Unless you have an extremely tail heavy plane, where you need to cram every last component up front, there is no need to.

I prefer a nice and tight direct connection to the carb with golden rod back to the servo. Having slop in the throttle linkage doesn't shield your servo from vibration; all it does is give engine vibration something to act upon. The airframe transmits just as much vibration, if not more, than the linkage does. I would advise against micro servos like the HS-81 for throttle duty. The gears, motor, and potentiometer are that much smaller and more fragile. The weight saving vs. a standard servo is miniscule.