Well the proposed marine rules already include a way to stop a squat giving his stuff to a marine or the other way round by saying when you buy a piece of kit for a marine is "astartes" kit and can only be used by astartes. The kit thing for other examples is always going to be hard unless we set sizes that have no game effects for example
Squats and grots = small
Human and eldar = medium
Ogryns and sslyth = large
And when a piece of armour or a weapon is brought you say what size it is and then it can only be transferred between models of the same size, just off the top of my head spit balling
I'm unconvinced that Sslyth power armour would fit an Ogryn, however, the easiest option I've come up with is to give some items this:
Tailored: This item is custom fitted for it's owner and cannot be used or worn by others. A character with Armourer, Weaponsmith or Inventor may attempt to modify it for another user by taking an initiative test instead of working a territory. the test is automatically passed if the warband controls a Workshop/Weaponsmith.
Then we can just use species/usable by and maybe origin to rule out characters from using stuff.