Hi there, I'm Dr. Sara. I'm a licensed veterinarian who works exclusively with cats and dogs. I'm also owned by two lovely kitties who challenge me daily and teach me plenty about being a cat mom, haha.
It's not unusual for kitties who used to live outside to have trouble integrating into a home environment. Cats can be social, but they are hard wired to be loners when they're in a "wild" state. If she spent 6 years outside going it alone, she might have real difficulty "warming up" to your other cat and living inside the home.
Cats are persnickety critters and their social cues can be subtle and complicated. It's best to try to continue your routine as "normally" as possible and make sure you continue to give the same amount (or more) attention to your current cats even when the new cat is present.
It's best to keep the new cat separate from the resident cats for at least a week to start out with, which I don't know if tried. If she's really going after your resident cat, I'd separate them again and aim for a slower reintroduction. During this time, your new cat's scent will be around and your resident cats can get used to her. By keeping the new cat in one room, the resident cats can familiarize themselves with her scent but not have to be face to face with her. If she's been chasing your resident cat, this gives your resident cat a break from having to sneak around and be worried that she's going to get chased. After a week or two, you can start the formal introduction process by bringing her out of her room, whilst allowing your resident cats to go into her her room (without actually meeting her). This gives her a chance to explore the home without being followed or frightened by your resident cats, and it allows the resident cats to get a good sniff of her environment. Once again, the point here is to allow them to familiarize themselves with each other's scent and presence without actually having to come face to face. You can also exchange bedding and/or litter boxes during this time for the same purpose of familiarization.
You can move on to allowing them to interact through that screen door for awhile until there's no evidence of adversarial behavior. A good thing to do here would be to have meal times and engage in play in the general area of the door so that they learn that they all get something great (food or play) for being around each other in a non-threatening way.
It's a flexible process from there, depending on how they all interact with each other. If they seem to get along well, then introduction periods are shorter. Engaging them all in play at one time can be helpful - something like getting them all interested in a laser pointer or a special toy. This is a great ice breaker. If there's any tension, then I'd keep them separate while you're not around until you're confident everyone seems to be OK. They will need some freedom to set up their own hierarchy and social group, so unless there's signs of aggression, I try to just treat them all equal and allow them to work out their social status on their own. It's also best to be sure that each cat has their own personal space so that they can eat and drink from their own dishes without needing to interact with each other. This helps diffuse territorial and resource related aggression.
Marzipan may still want (and possibly need) some outdoor time to keep her on an even keel. It can be very difficult for a cat to transition from the wide open outdoor world to living in a home. Sometimes just allowing her access to the outdoors can significantly decrease her adversarial behavior inside the home. If that's definitely not an option, it might be a little tougher to get her to integrate. While most house mates won't actually harm each other physically, they definitely can cause each other a great deal of stress if they're not getting along well. In severe situations I have prescribed anti-anxiety medications like fluoxetine to both the aggressor in the situation to calm them, as well as the cat who is being chased, to embolden them and help them feel more at ease. Herbal remedies like Composure Pro, Feliway diffusers or calming collars can be used on all the kitties in the house to help bring down the stress level in general.
This can certainly be a difficult process with no one single "Cure all" - there's definitely some trial and error involved in finding what will work best in any one given household and group of cats. Please let me know what other questions I can answer for you.
Here are links to the products I referenced:
Calming Collar: http://www.petco.com/shop/en/petcostore/sentry-calming-collar-for-cats
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