Mike,
I think your concern about mixing of R134a in Freeze 12 with your system is a valid one.
check out this link:
Auto A/C
PANTERA INTERNATIONAL
A DE TOMASO CAR CLUB
Presents the De Tomaso Forums
The On-Line Meeting Place for De Tomaso Owners and Enthusiasts From Around the World
Clicking on the banner will take you to the sponsor's website.