zigmo36 - You shouldn't have any issues! We are running Nortel/Shoretel hybrid without any problems. Just don't expect the Shoretel to do everything the Nortel could. The biggest let down with the Shoretel is the Routing, the system wasn't to try to be smart enough to do the routing for you, so you have to trick the system with Least Cost Routing.
Follow the guide that KCF... sent, There are a couple of guides floating around with like this and I didn't read this one all the way.... The one main point is don't fool around with the link between the two systems, and just go with with ISGF QSIG from the start!