Hi .. unfortunately with PIX devices on transparent mode, you are limited by the ammount of physical interfaces it currently has ..i.e A PIX with 6 interfaces will allow for the creation of 3 contexts of 2 interfaces each for protecting 3 segments.
If you already have a Cat 6500 I suggest you to consider the possibility of implementing a FWSM ( Firewall Service module ). With the firewall module running on single mode you can create a maximum of 8 bridge groups ( 2 interfaces-VLANs per bridge group) to protect 8 segments. Or if you prefer you can use multiple context mode, create one bride group per context and protect as many segments as you need.
Please refer to the belowlink for more information about FWSM and transparent mode.
http://www.cisco.com/en/US/partner/products/hw/switches/ps708/products_module_configuration_guide_chapter09186a0080577c38.html#wp1201980
I hope it helps .. please rate it if it does !!