Get Your Cat Care Questions Answered By Cat Vets ASAP
You need to keep the new kitty separated from the existing household members for a while to make certain it is 100%. You don't want your current cats getting ill if this baby has something going on. Confine it to a single room in the house with litter box, food and water. Your other cats will smell the new addition under the door, so this will help initially. After 10-14 days, if all is well, put the newcomer in a carrier and set it in the middle of your general living quarters. Allow the cats to greet through the cage to become familiar. Do not leave them unsupervised until you are 100% convinced that all is well. It sould take several weeks before your current pets accept this newcomer. Be prepared for a few spats with a water gun or spray bottle. If they get into a fight, a quick squirt will break it up safely.
Don't get discouraged if things are a bit rocky to begin with. It will take time, but everyone should settle down fairly soon.
Please let me know how things are going!
Wow- you do have a bit of a situation on your hands. Having worked with cats for many many years, I've found that they tend to do better in even numbers (ie 2 or 4 cats get along better than 3 or 5 cats). When you have an 'odd' number, one cat always seems to be ignored or picked on, and it doesn't necessarily seem to be the new addition. You can try purchasing some feliway plugins to put in the house. They put out calming phermones that have a tendency to help mellow out cats. They're odorless to humans, but do seem to help with behaviorial issues. You can also purchase a squirt gun or spray bottle that puts out a stream rather than a mist, and if Lucy gets nasty with Papi, you can squirt her with water. Usually, a few times of this will help stop the behavior. She may not be loving and friendly toward Papi or Cleo, but she should stop the attacks. She will associate the getting wet with negative behavior, but will NOT associate it with you doing it or with Papi himself.
I'm also assuming that everyone in the house is spayed or neutered. If not, that is the first thing to take care of. The phermones that unaltered cats put out can be very intimidating and problem causing.
Lastly, if none of the above seems to be helping - and it may take a few weeks to turn things around - you might call your vet and ask for a referral to a veterinary behaviorist. They can be excellent at turning situations like this around and are usually very reasonable in their rates. They may ask to come to your home and watch to see how the cats interact or just see you in the office.
I wish I had a magic switch you could flip to turn these guys into a happy crew overnight, but it is going to take some work, especially since they are all adults. Good luck and let me know how things progress!