Yes, those are the only ones at this point.
And on that note, it's perfectly fine to make third party skins, even commercially. Legally, it's basically the same thing as forking the code and building a new game, which is, of course, also fine.
As for my "share," I'm going to have to recreate the enemy definitions and levels, as those are also part of the GPL XKobo code (and don't really fit the new physics all that well anyway) and I might put that data in the proprietary data. However, I still intend to package the FOSS source tree as a fully playable game, so in that case, it would just have a conversion of the XKobo/Deluxe levels or something - so essentially what is there right now.