RE: 6 volt battery for rx of xp6102?
Rich, to my knowledge all rx's from jr/fut/hitec/airtonics since at least narrow banding in 1991 have been able to use at least 6v packs. The jr 6102 would have come with the r700 (ppm) or r770(pcm) rx. Both of these I have used with 6v with no problems or warnings from the distrbutor. JR themselves was making 5 cell rx packs long before 2.4ghz came out.
Again, servos would be the question- not every servo out there is rated for 6 volts. If he is using standard servos though, I.e. jr 537, 517, futaba 148 or 3003 or the like then he is good to go. 6v is a great way to boost the performance of servos as well as provide protection if one of the cells in the pack goes bad.