Hi XXXXXXXXXXX~
Unfortunately, Chihuahuas in general seem to have a hard time delivering pups..and with the fact that she was bred by a bigger dog could absolutely cause the pups to be too big for her to deliver the old fashioned way.
The only thing you're going to be able to do is to keep your eyes open for when she goes into labor, and at the very first sign of trouble (straining, no pup after an hour or two of active labor, ect.), you'll have to get her in to the vet for an emergency c-section.
Maybe she'll get lucky and the pups will have small Doxie heads, but you should be prepared for the possibility of her needing surgery to deliver the pups.
I hope this helps.
Certified Veterinary Technician
Associates Degree in Veterinary Technology. State and Nationally certified.