it may be possible to use pix as a interVLAN router but it is highly discouraged. (cisco or good netEng would say NO!)
among other 'cons' of using a pix for this are:
* administration - high maintenance to setup pix for all vlans access to/from each other
* packet inspection induces latency
* must specifically permit deny all required ports (TCP/UDP) let alone other protocols
* must use dmz interfaces for more than 2 vlans
there are many more reasons this is not a solution for your needs.
consider using an inexpensive L3 switch such as 3550 with basic routing or even a 2800 series router. (or even a 2600 router gotten cheap from ebay or the likes)