as long as you use a 6V pack (or 4.8) then you're safe, any higher than 6 and you may damage your servo's and receiver. servos can usually cope with 6V, and this is where they operate at their best (more torque and faster). mah is how long the battery lasts, anything can cope with any mah, but obviously the higher the mah the longer it will last before going flat. as sammy said, you dont need much more than 1100/1600mah for a good days bash with nitro's - so no point in having a huge amount of mah if its just going to make your car slower but with electrics you may only get a 5min run time. so thats why they opt for more mah, and need bigger cells to cram it in.