Not sure i understand exactly what your first question is asking.
You can have multiple FWSM's in the same 6500 chassis and you can have an FWSM in a pair of 6500 chassis. The number of MSFC's ie. supervisors does not affect the FWSM.
Yes it's possible to migrate ack and forth. Be aware if you go from single to multiple then the FWSM uses your existing config to create the admin context in multiple mode. Transferring back. Well if you have multiple contexts you need to be careful as obviously these cannot all be used in single mode.
There is a field notice to do with Distributed Etherchannel and the FWSM which can have an impact on performance although there is now an IOS that fixes it. See attached link:
Yestreday , I have configured a 6509 with a FWSM card, I verified all , trunk, redundancy, and context today. and it's work except a problem with a multiple context with 2 outside interface who share the same svi.
But I asked the question in the firewall/security category.
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