Dan,
I have run both together for quite a long time with no issue. It was great to have both as I made the transition from ADDT to WebAssist. I still use a few ADDT behavior such as conditional regions and such which should not break no matter what upgrades are made to PHP, but I have successfully shifted all the heavy lifting to WebAssist products with no issues at all.
My machine is MacBook Pro with Snow Leopard.
Regards,
Brian