The A123 Sonic charger can only charge up to 3S so you will have to use another method if you want to make a 6S pack. I currently use a 4S pack and 2S pack in series.
The M1 cells should be charged with the cccv method. A Lipo charger will work but you will have to manually monitor the voltage and pull the packs off before they reach 3.6 volts per cell. The LipoDapter allows you to charge up to 8AMPS and cuts the charge off at 95% capacity for the M1's and 85-90% for Lipo's. I got my M1 packs and LipoDapter from Pete at
www.modelelectronicscorp.com