Yes, you can mix HR92 and HR91 on the same Evohome system - I had an HR91 for evaluation on my system for a while, however I didn't keep it as I didn't like the performance of the HR91 nor the compatibility with my TRV valve bodies.
As for preventing the screen being ripped off (which my 3 year old has done twice) I found a reasonable solution is to remove the triangle wedge so the screen can sit flat then put a loop of regular clear cellotape across the screen and around the entire body - this keeps the screen firmly locked down flat so it can't be lifted yet is invisible from a distance.
I've done this to several of my HR92's now that are in "vulnerable" locations and no more mishaps since then...