I had Lucy since she was 4 months old & when she was 4 years old I got Pebbles. Peb's was 9 months & is mixed yorkie/doxie.
When I bought Peb's home, she came in wanting to be the boss. Lucy didn't like any of us for about the 1st week or so. That was back in July. Now they have so much fun together, it's a blast for them to be able to play all the time. Peb's still tries to be the boss & Lucy lets her up to a point.
It takes time for them to get used to each other and it can work out.....good luck.