There is some compatibility with the American Girl (and possibly Wellie Wishers) series. The Monster High characters can wear the hair, skirts, and glasses from them, and I'm sure vice versa. Otherwise their appearances are strikingly different. The parts ARE compatible though, like you could fit an AG upper torso on a Monster High lower torso. It looks very strange though. xD With the correct figures from a variety of lines, however, I'm sure that some stocky Monster High characters could be achieved (like the lunch lady). I can't imagine which characters you could use for that though.
I have plenty of extra MH characters though, and I plan on using hair and other parts from the AG line to make a bunch of generic Monster High students. Should be fun, if I ever get around to building their school. x]