If the servos are the typical medium priced ones, then you can get by with just about any 4 or 5 cell NiMh or NiCad battery. You could also use a 2 cell LiFe but realize that will require a proper charger for the LiFe. For many years, we all got by with the 4 cell NiCads with no problems using the Futaba or Hitec standard servos. Now that many are using 2.4GHz units which are far less tolerant of microsecond voltage drops (loose lock and have to reacquire signal) many have gone to the 5 cell batteries or are using ESC's which put out a typical 5 volts to the servos. You should be able to get by fine with 700 mah batteries, in fact the 500 mah would give you several safe flights before recharging. Now, if you are using the newer digital servos, they demand more power so use the larger mah batteries for them.