Male/Female are supposed to be the best match because of the natural 'chemistry'.. then male/male...
If your boy is fixed, then I wouldn't worry too much about him humping her. I had Oscar first and then brought Ruthie home (Oscar was neutered before she came along) - he hasn't humped her once. She humps him sometimes though
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It really depends on what you prefer - bringing any new dog into the house could result in your male changing his behavior suddenly and starting to mark in the house etc. You just never really know.