Czuch Chuckers: I wouldn't consider Dark Chess as a chess game. It really is different in the type of thinking involved to play it even though it has the less rules changed of all the variants (Just two). Games like Janus Chess are the same thing as Chess when it comes to game types and the thinking involved. I can see lumping those kind of variants in a group for the type of rating comparision that you're talking about. Dark Chess is more like Battleship or Stratego played with a Chess set. This is why I'm able to hold my own or win in Dark Chess against some very good players of regular Chess or similar variants where the same person would clean my clock without trying too hard.
And then, what about variants like Atomic Chess? The way capturing is done it that game, and how one attacks and defends his position is nothing like regular Chess and certainly should have its own category. Perhaps you can give it some thought and come up with how to group the games if you agree with me on this? If you think all Chess variants are just playing Chess, I'll say I disagree. Does being good at Pente, make one a good Five in line Player? It would seem so. I'd put those games in one group. There's already a grouping of the games by type on this site. Perhaps it could be used for the rating that you're talking about. The Chess games could be broken into three or four groups and the rest of the games counted as a group. How'd that work for your groups?