There are three main criteria:

1. The charger is designed to charge lithium cells.
2. The charger can be set to charge the appropriate number of series-connected cells.
3. The maximum charge rate does not exceed C, or the capacity of the pack.

550mAh is less than C on both Etec 1200 and 700 cells, and from the sounds of it, the charger is designed to charge three lithium cells in series. It should work fine, though I'd keep an eye on it the first couple of charges to make sure the voltage doesn't exceed 4.2 Volts per cell, or 12.6 Volts.