What is the max safe temperature for a nimh battery???

That's the easiest - just wire them so you have a plug that'll plugin the batt slot in the rx. This drains a little away from the servos, but unless you get a power draining fan, it shouldn't be noticeable.

If you're runnning nitro, its for a batt. If you have a 3rd channels in the rx you could you use that if you were running nitro - all the slots have power.