Not to get technical... Okay, I'm getting technical:
You can, actually, use a 1911 mag in a P-90, you just have to hold it in with your off- hand in the dreaded "teacup hold" as you shoot. I have done this when my foolish friend forgot to bring a 2nd mag for his P-90, and complained that he could'nt reload for informal match shooting.
Not exactly worth the trouble, though.