
I think I'm right in saying that the two coats of chihuahuas are different breeds and shouldn't be crossbred or the KC won't register them? I'm sure someone with more knowledge than I have will confirm or correct me! :)
I have a book written by an experienced chihuahua breeder who says that you can tell if a pup is going to be smooth- or long-coated (this was in the days when the two coats
could be mixed, which is why you still get either in the same litter) almost as soon as the whelp is dry, because a long-coat will have a ridge of hair along the back of its neck down past the withers.
As for age, many toy breeders don't home their pups till they're about 12 weeks because they're so very tiny and fragile.