You can't specify the port the PIX will use specifically, unless of course you use a static then the ports are left the same. The PIX PAT's source ports as follows:
- If the source port is TCP/UDP 1-511, then the PIX will PAT the SRC address to one in that range.
- If the source port is TCP/UDP 512-1023, then the PIX will PAT the SRC address to one in that range.
- If the source port is TCP/UDP 1024-65535, then the PIX will PAT the SRC address to one in that range.
You could set up a filter in your IDS to just ignore any port 69 from the PIX's PAT address.