Finding homes for cute little dogs is never a problem. You won't be stuck with any of them.
He is refusing to mount her, but she wants to mount him? I would venture a guess that he is young and hasn't sired a litter yet? Or not a dominant dog? Usually males are more than willing unless they are young or shy. You may have to place him on top of her while holding her collar to encourage him to do his husbandly duties. Even if they do not tie this time, eventually he will grow up and be more than willing to service his dam. Some dogs are just shy or just young and not ready to mate yet. If she is mounting and flagging then she is close to her fertile period. If it doesn't work out this cycle, you will just have to wait until another cycle.
You could get both long and short puppies from this pair-- long hair is recessive in chihuahuas. You won't get merles (obviously), and doubtful you would get black/tans or diluted coats. I don't know anything about the color genetics in poms, but if they were both chis, I would look for roans and brown puppies. I don't think you will get fawns, because of the darker markings on the female, but dilutions are recessive, so it might be possible, but doubtful.
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