A freshly charged "6V" NiCad is about 6.7 Volts. Most servos don't mind, and the manufacturers will tell you when a particular servo does not work above 5 Volts. I've used 4 and 5 cell NiCads for many years, a very proven technology that is super easy to maintain in a always ready to fly mode with the 1 hour timer method.

But NiCads are on the way out, as too toxic, and low energy density. They had a good 50-60 year run, though they started out as crap when they were configured in button cells.

So today, the LiFe battery technology is looking pretty good in receiver applications when you are not flying LiPo powered planes. You can also use a two cell LiPo, but then you have to also use a regulator unless your airborne pack is configured for high voltage. Most are not.