I've asked Pyro about what is considered an acceptable skin substitution for the various Yaks. There are no permitted substitutions for the Yak-3 and 9U. You can skin a Yak-7A on the 7B, but a 7B with the cut down rear fuselage is not allowed. Now that the Yak-9T has a 20mm option you can skin any of the early Yak-9s on it, including those with the forward cockpit position (9, 9D, 9DD etc.)
I'm going to do a few Yak skins but haven't decided which yet. If anyone has any particular Yak skins they want to do, please post which ones in this thread.