new, larger dog mounting smaller dog already in household

Question
Hi, We have a 10-lb Pom and just had a 5-lb teacup Pom who died at 16 years.  The 10-lb is 8.  They got along great, and the older one was happy to have the younger one be the alpha.
We just rescued a 15-lb, neutered Pom, who is also considerably taller than the 10-lb. His owner died and he has been staying at a vet or in foster homes.  He is perfect, except that he has started mounting the 10-lb who is upset. (He's neutered too).  He is also doing it to the cats, but they have put him in his place.  I know it's a dominance thing. How can I change the new guy's behavior so that my 10-lb dog does not feel dominated?

Answer
How do you know the 10 lb dog is upset? This is an important question.  Are you projecting or is he vocalizing, showing fear, hiding, etc.?  That dog knows very well what's going on.  He may not like the fact that's he's being dominated (change of role for him), but if he's responding normally this "problem" should be very short term and self extinguish.  In the case between these two dogs, it appears that the resident may very well be a candidate for demotion by the new comer, whether or not you agree.  Interfering may cause more problems than it's worth.  If your older Pom is really "upset", I need you to explain further so I can help you.