OK, this is actually a HUGE topic, with multiple layers of different answers both depending on what you are trying to achieve and how complex you are willing to go. Because of this, the answers to this topic need to be separated into sub-answers.
I will cover the most basic ones first, and then add topics as time permits.
- How to Rewire Reason into Pro Tools
- How to use a MIDI controller with Reason and Pro Tools
- How to control individual levels of devices in Reason through Pro Tools
- How to use ReDrum as a drum sound module for Pro Tools
- more to come…