Hugh Hefner's brand new girls next door, twin sisters Karissa and Kristina Shannon, have turned out to be truly bad gals . . . with the police record to prove it.

But it seems that the prospect of bedding down with a couple of sexy criminals only further stokes Hef's fire! According to his beloved--and recently split-with--original girlfriend, the delectable Holly Madison, Hef isn't wasting any time mourning the loss of her and her fellow Girls, Kendra Wilkinson and Bridget Marquardt.

In fact, Holly recently told US magazine just how much fun Hef's been having with the Shannon sisters!

"They're a lot of fun," she told Us Monday at the 2008 "Bowling for Boobies" bash in Hollywood. "I would never try to personally pick Hef's new girlfriend. I would like him to take his time and actually get to know somebody -- which he never does! But as long as he's having fun, I'm happy for him! He's having lots of fun with them -- and a lot of other people!" she added.

Despite the girls' criminal backgrounds (they pleaded no contest in August to felony battery and were ordered to pay restitution), Madison, 28, told Us, "I think that they're in a good place right now and hopefully will stay out of trouble. They are a little young and definitely like to have a good time," she went on. "I just hope that they can take this new fame with a level head."

A level head . . . and their adorable, bite-sized original boobs.