Is the FWSM for the 6500 hot swappable? Should I be powering down before inserting or removing it?
I have seen conflicting documentation...
Yes, you should be fine. One of the things you are paying for with the 6500 is the ability to hot-swap modules without powering down the entire switch.
There is always the remote possibility that the switch could crash so if the switch is critical i might do it out of production hours (but i still wouldn't power the whole switch down).