Oh, you are working with some old stuff there! So in the charger there should be a setting for NiMh/NiCd and it should have a cell count. I don't have my S6 iCharger setup so I cannot look at it right now, and I no longer own the 306b (wish I did.... never should have sold it).
For charge rate, 1-2s should be fine, just keep an eye on pack temp as they do get hot. I remember charging at 5-6 amps on a 6 cell Sub-C pack and I had to run fans on the packs to keep them from over heating. I'm talking "Blowiematron" fans lol. Loud and moved a TON of air.
Dangit, I wish I had my S6 ready to hook up... Never even plugged it in yet.