I think the most important aspect here is: Supervise all contacts between both groups of pets.
In my experience it doesn't matter which pet was where first. What matters is how each individual reacts to the presence of another individual. Some dogs are curious and careful when meeting a cat, others immediately have their hunting drive triggered. Some cats stand their ground, others run away at the mere sight of a dog.
My advice is to divide the house into 3 parts:
- One set of rooms where the dogs live
- One set of rooms where the cats live
- One room of "neutral ground" where they meet under supervision
The neutral room should have some furniture with raised surfaces like a table, sideboard or shelves, where the dogs can't reach and cats can flee to from the dogs. It can also help to barricade the door towards the cat's part of the house with a baby grate or something similar. The cats should be able to jump over the obstacle, while the dogs should not. That way you can offer the cats the opportunity to meet the dogs without forcing them to stay in the room.
During at least the first week (maybe 2 weeks), let both groups of pets explore the neutral room, but only one group at a time. That lets them get used to the smell of each other without the potential for conflict. It makes them understand that there's someone else frequenting this room and living here.
Then start the meet and greet one pet at a time. First let the most calm and docile dog into the neutral room and offer the same opportunity for the most relaxed cat. Stay in the same room and be prepared to get them back into their respective parts of the house should any problem arise.
Repeat the same process every day, mixing and matching individual pets. If all goes well, you can then start letting several pets into the room and eventually open the doors to let them roam wherever they want.
If a dog doesn't stop chasing a cat, you need to teach him (let's assume it's a male for brevity) to endure the presence of a cat without chasing it. Put the dog on a leash and lead him into the room with the cat. If he starts chasing, physically turn him around so his rear faces the cat and his head faces away from the cat. That does not mean "put him on his back" but "turn him away from the cat". This way you signal him that you don't want him to chase the cat. Reward him with treats every time he looks away from the cat.
If a cat is too afraid of the dogs, offer him (again, let's assume it's a male for brevity) a safe place where the dogs can't reach. That can be a room where the dogs don't go (a baby grate can be handy) or the top of some furniture or a cosy windowsill.
Make sure you have enough litter boxes for all cats. The general recommendation is one litter box for each cat plus one extra. Don't place food and water bowls next to any litter box because the cats will avoid drinking this water, which can lead to urinary tract diseases. Make sure at least one food bowl, one water bowl and one litter box is very close to the cat's safe place.