VinylBoy:
I don't think they're selectable. I've got nearly every issue of Neo Geo Freak (I missed the first four and the sixth issues), and when they were covering MOTM over the course of three or four issues, the move lists didn't have anything on Rimeruru or Karin. They had rundowns on the secret characters in the second half of the coverage issues(Geese, Evil Ryu, et. al.) and it seemed like a comprehensive list when all was said and done. I don't think they would have left Karin and Rimeruru out of it. Furthermore, the empty squares on the character select screen all fill up as you unlock the secret fighters. After Orochi Iori and Evil Ryu the squares are all taken up and subsequent completions of the game don't give you any other characters hidden beneath tiles to unlock.
Sure, there may be secret slots off to the sides and Rimeruru and Karin might be selectable under the conditions you suggested but I figure that by this time we'd have heard something about it.
Taiso