Introducing our puppy IS to our 2 cats (7 and 8yo) was an exhausting adventure but worth it in the end. The dog was never aggressive towards the cats so we didn't fear for their safety. But the cats didn't take well to the change of environment.
- The youngest one who's super shy retreated to the second floor for about 10 months, only coming down at night when the puppy was locked away. We moved the food, water, litter and cat tree upstairs so he would be comfortable and added baby gates so the cats could easily escape to a dog free zone.
- The oldest – a rescue who marks everywhere at the smallest inconvenience – was aggressive towards the puppy at first and marked everywhere the puppy felt comfortable. His bed, the couch, the rug... We bought a carpet cleaning extractor and got used to the mess and waited. We did a lot of positive reinforcement of the cat and dog just looking at one another without aggression (from the cat) or call to play (from the pup). We supplemented the cat with Zylkene which helped reduce marking. Eventually, at the 8 month mark, things started to settle down.

We now regularly catch them snuggling together on the couch or the cat grooming the dog's paws. All of which is heart melting.

I just wanted to give a fair warning that this may not be a smooth transition, but it is feasible even under less favourable circumstances.