I have the Mercury PBox and the Xicoy LGC15 installed in my larger T-1. I use the Mercury gyro on the three flight control surfaces and the LGC 15 operates the gear, brakes, nose door and steering. Works just fine. I have not seen any conflict with two gyros working on the steering, even with the Rudder and Steering channels mixed.
For a mini, I would probably not use a PBox. There is not a lot of room and the lighter you keep the plane the better. If it was mine, I would also opt for the lightest turbine that would still fly it. When we were working with one brought to us this past summer, I kind of thought maybe something smaller than an 85 would work better. Maybe a 60?? But I have not seen one with a smaller engine, either. From my experiences with one this past summer, this is just something I would be considering if I built a new one of my own.
The mini with any turbine systems in it has relatively high wing loading. When building it, I would pay particular attention to keeping it light. But, one place to add some weight is to beef up the gear mounts. Also pay particular attention to the CG. The plane is relatively unforgiving when the CG is off by much either way. Just my opinion after flying one a couple times, surviving 3 snaps and struggling for hours trying to get one lightened up and dialed in.
Also, ... In my opinion, it is NOT a trainer, by any means, no matter what anyone tells you. There are a lot of other jets out there that are much easier to fly, see and land, especially for a rookie!