You have to be carefull with overlapping NAT/PAT statements within your config. PIX accepts it and this is good, but the problems you will be getting is with the returning traffic (in most cases). You have to make sure that returning traffic is not translated.
Conclusion:
Yes, you can have overlaps
Yes, it can cause sesious trouble
and Yes, it can also work with the proper configuration.
Kind Regards,
Leo