Yes, you can mix. The trial and activation systems are separate.
A user needs admin priviledges to start a trial when TA_SYSTEM is used for the TA_UseTrial()-call, right?
I'd like to activate trials silently, so I'd rather use TA_USER. But later when a user activates then using TA_SYSTEM makes more sense and requiring admin-rights is acceptable then. So the question is: Can I mix it or will that cause trouble?
Yes, you can mix. The trial and activation systems are separate.