What is the current draw for a Ch3-R hexapod with standard servo’s?
About 1 amp standing and from 6-8 amps when walking.
I use the CH3-R with 6x HS-475 for the coxa and 12x HS-645 for the femur and tibia. Further 6x Led’s for the leg light kid powered via the SSC-32 servo controller. Is there any special thing I must keep in mind if I using this with a 6V 4600mAH battery?
You can think of how much longer your bot will run than most everyone else. 
Beyond that, there aren’t any downsides really, just weight.