Bella came into a house with 2 adult cats. The cats stayed in the bedroom solidly for 2 weeks, only coming out when Bella was asleep in her pen and then they'd come and stare at her very warily.
After 2 weeks, the older cat (who can be a bruiser) decided enough was enough, and she came out, walked very gracefully around each and every room, finishing in the kitchen, where she sat in Bella's bed to clean herself (making sure Bella was on the floor paying attention), and then drank from Bella's water bowl. She also decided to lay down some ground rules with Bella and these mean the number one rule is NO CHASING or you'll get a nasty swipe, and number two being "You will submit if I walk past". All of which Bella has heeded and she will not chase Ziggy, or go past her into a room, and frequently submits to her. Now, 7 months on, they kind of get on, Ziggy will go and lie down about a foot from Bella and Bella can usually stay calm enough to stay where she is. She still hurls herself around in what we call her court jester act but Ziggy just hisses at her or ignores her.
Taj is another matter (currently gone missing since Monday night
) as he has never stood up to Bella so she chases him through the house. He has gradually got used to her and he will now come and sit on my lap while Bella's on the floor at my feet, and he basically just stays up high out of the way as much as possible. We are trying to stop the chasing by calling Bella very loudly as SOON as she starts after him, and if she stops and comes back she gets a treat, so this is starting to work.
This has taken 7 months so far so just give it time, make sure you give the cats as much attention as you did before (which can be hard with a young pup, I know), give them nice dinners, and also I think what helped with us was to feed the cats first, in front of Bella (who would lie down and wait for them to finish), so that Bella knew her place in the pecking order was below the cats. We still do this now and Bella will not even bother looking, she knows what the score is.
Good luck!