My groups almost always have characters from all three FFG Star Wars systems. This doesn't cause problems, and everything seems reasonably balanced...except force powers that allow force dice to skill checks (e.g. the manipulate control that allows FD to be rolled with mechanics checks). A force user with a force rating of 3 or higher tends to be better at skill rolls when they can apply FD than other characters that have invested more heavily in those same skills. When the party needs a critical roll made, it almost always makes more sense to have a force user do it, because they can always take some conflict to get the necessary successes and/or advantages. The Enhance tree give FD to Athletics, Resilience, Coordination, Planetary Piloting, and Space Piloting for 30 XP.
This doesn't seem to matter much at lower XP, but once the party is over around 500, the disparity starts increasing quickly. Does anyone else find this to be a problem? If so, has your table instituted any house rules?