So, RevBob was right back then, but his statement wouldn't be right it playing with new cards. See the last replay here
http://forums.sjgames.com/showthread.php?t=70123
Now both Half-breed and Super Munchkin state on the card that they can work with with one race or class respectively. Second sentence in the card text. Totally depends on when y'all's super munchkin cards were printed, but half breed cards have always said they work with 1 or 2 races afaik.