either method will work
of course you'll want to charge them all at once
think about it , all the other NiMH battery packs are just a bunch of rechargeable AAs
make sure you use the same type of batteries
as for amperage and voltage - charging amps should not exceed 1/2 the amp hour of the batteries and voltage should be like 2 volts higher
just look for any NiMH charger that can handle a 6V 2 AH battery pack , and that will work