The Futaba S9351 servo is a new one. It's slated for release in November or December. We hope to have stock shortly after its release.

Most of other manufacturers' servos will work with Futaba equipment, but we'd not know which ones will or won't.

When servos are rated at 6V, that means that they can be used with a 5-cell NiCd battery pack. As long as you have no more than 5-cells in the pack, you should be OK.