The issue of low voltage only affects one brand of 2.4 radios, JR/Spektrum. Futaba and Airtronics have the low voltage cutoff values low enough that it's not an issue with those radios. And even for JR/Spektrum has addressed this issue and it only affects their older 2.4 radios.

Most all servos will work just fine with a 6 volt battery, but not all so make sure you check that the servo can handle it.

Ken