Hello! I've no advice but can share my experience.
Earlier last year, I brought in a 9 wks old female GSD into my home that already has a 9lbs male peke mix, so somewhat similar to your scenario. Same thing - peke hated the puppy to the bones the first couple days. 2nd week, peke stopped acting shocked and was normal himself but would avoid the puppy. The puppy just wanted the peke to play with her. Over the next few months, the peke began to accept the puppy, would let the puppy lie next to him but still showed disdain. Fast forward to today, they are best buds! They play all the time. The GSD has resource guarding issues around outside dogs but will not guard with the peke. The peke will go and lean next to the GSD. They would huddle together when scared about something. They both have full house privilege when I'm gone and they would both go to the same room together till I'm home.