There should be no problem running that servo. Servo's do not care what supplies them the power as long as it is within the operating voltage. And that is controlled by the ESC or the BEC if you install one. Some servos have maximum voltage ability. Most will run fine at 6 volts. Some car use higher voltages and usually those are marked as HV servos such as say this Hitec:
http://www.hitecrcd.com/products/digital/hv-ultra-premium-digital/not-set.html
it can operate on up to 7.4 volts. As in the manual but may be able to use higher. Most servos are meant to run on between 3 and 6 volts. They usually give you 2 specs, one is 4.8 volts and the other 6 volts. The higher the voltage, typically the higher the power it can achieve.
What actually powers the servo is output from the esc. Yes it does run to the receiver first and then routes to the servo through it. But I have never seen a receiver that changes the output voltage of the esc's servo power amp and voltage rating. They allow the power to run the servo to pass through from the esc but add in the directions or control from the radio. In other words the receiver only routs power without changing it from the esc and adds in directions on what to do (stay centered, turn, etc). So regardless of what type of power source you use (NIMH,NICAD,LIPO,ETC) the voltage is adjusted to approximately 6 volts (from most esc's) and that is what gets to the servo for power. It could be Cow Manure that is providing the voltage, it doesn't matter.
Now what is causing heat can be a few things. One is that the servo you mentioned is not very powerfull. It is rated for about 130 ounces of torque. That is marginal for a crawler. So it will strain trying to move the tires. That will cause heat. The other can be if the servo is trying to push the steering further then it can. This will cause the servo to really overwork. A good test for this is to hold the crawler off the ground. Try moving the steering full to one side with the radio. If you see that the tires stop turning but the servo still moves or continues to swing then that needs to be fixed. The radio you mention does not have the ability to adjust the end travel of the steering. That is one of the features you get with higher end digital radios. The ability to adjust how far the servo travels. So you have a few options. One is to not turn the steering wheel full (bad idea and hard to manage). Turn down the steering rate (affects both left and right travel) or upgrade radio system to get needed features. You may be able to do other things as well such as using a shorter servo horn as it will not have as much servo throw and may keep the steering from reaching it binding point.
The servo will get warm, but should not be hot. So if it is only getting warm, then you may not have too bad of a problem. But if it gets hot as in "Ouch, I just burnt my finger" then it is straining too hard. I think if you look into making sure the servo is not over throwing and binding (most common cause of this whole issue) and work on that, then you should be ok.
*EDIT* Benoj beet me to it and put it in less words. I have a bad habit of that.
