My problem seems to be adding new recruits to an already established military. I've got a half dozen Legendary Wrestlers/Armor User/Shield User/Hammerdwarf champions for a melee force. They've got ten years worth of stat increases, and between 20 and 50 named kills each. When I add a couple new recruits to the mix, I strip the veterans of their weapons and deck out the newbies in plate mail, but there seems to be about a 50/50 chance that the green soldiers will get a spinal injury from wrestling. I've started putting the vets on duty while recruits train up.
Notes on the next release say that sparring will be changed to a leader-lead squad activity. I'm looking forward to that.
I have an easy solution for this. Every squad is made from a single immigrant wave. That doesn't mean I turn an entire immigrant wave into a squad, on the contrary, dealing with a squad of 27 soldiers with this clunky interface would be horrible, but if I want 10 more hammerdwarves, I draft ten of the worthless dwarves from the immigrant waves (you know, the soap makers, potash makers, animal caretakers, regular peasants - dwarves who don't bother to learn a
real skill). When squads get too thinned out, you can merge them if you want, by that time both squads will probably be full legendary. A legendary+10 and a legendary+5 are on equal footing, even if the older soldiers have more experience they're no more likely to hurt the younger recruits than they are their own squad members.
And always, and I mean always, give them at least chainmail before you give them weapons unless you're one of the psychos who sees surviving a training regimen as "weeding out the unworthy" (please, even Morul could be killed by RNG). Plate is up to your own discretion, I kind of like my dwarves wearing chain just because it's much lighter on the materials and actually, if you have a good leatherworker, can be nearly as good armor, better on some limbs.
Edit: rereading that, it seems like you were already doing that. In which case, I've never actually had your problem. Usually my dwarves spar with members of their own squad, so I dunno. Of course, I've only seen two spinal injuries from wrestling and one death total, so something else must be going on.