Only reasons I've seen for rejecting skins are -
1) Nose art would interfere with squad art.
2) Skin is not historically accurate (see below)
3) Aircraft model (see below)
a) As long as skins as historically accurate within the limitations of the skin itself (mirroring etc), there is usually no problem.
b) As long as HTC is not planning on doing the model e.g. Spit 8, they will accept skins for other model in this case Spit 9. One proviso, it must be outwardly very very similar.
Have a great pic of noseart if you want it.