Spoke to Skuzzy.
There are ceratin models they will accept different Mks, or variants skins on.
e.g. they will accept Spit 8 skins for Spit 9.
The criterion is if they don't think they will model the variant or Mk they will accept the skin.
It's still a question of you takes your chances.
Glad I spoke to him as I was going to do some skins but he thinks at sometime they may model it, so they wouldn't be accepted. NO I'm not saying which one!
Another criterion is they prefer skins that are 'different'.
E.g. he said a lot were not used because they were basically the same as others but with different squadron IDs.
If your still not sure give them a call, Skuzzy was very helpfull.
Hope this saves you some time.