Hi..I have three cats who are huge.
Izzy is only 2.8 and the cats run from 8# to 16# and she terrorizes them all the time and wants them to play and chase her.
They get a long fine now and when the cats have had enough they let her know.
At first they went into hiding for a few days when I first brough Izzy home but now they are all buddies.
Give it time and all will be good.