We have to combine MS/HS teams in the offseason just to have enough for 5 on 5 intrasquad. We do a lot of it, though. Keeps things more fun so they'll keep coming out.
I'm not opposed to the idea of it, though MS practicing with the HS team is a no-no here, and so is doing this kind of thing during the school year/out of season.
Anything outside of open field/gym and giving private lessons is not allowed here. I'm fine with that though. Most of our girls play volleyball, basketball, or both, and I wouldn't want to take away from their experiences with those sports (even if those experiences are objectively worse and most of them spend the whole fall and winter talking about wanting it to be softball season).