Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you today. I do apologize that your question was not answered before. Different experts come online at various times; I just came online, read about your situation, and wanted to help.
Now you definitely get a gold star for thinking about this before you decide to challenge Meg with a new cat in the household. Especially as this will be a major change and potential stressor for Meg since she has no experience interacting or sharing her territory with other animals and sounds to a less then dominant cat (so there would be a risk of a new cat coming in and bullying her). Therefore, it will be important to find the right cat to co-exist with Meg.
Now as I am sure you can appreciate that unlike dogs, cats are not pack animals (they are like little tigers, not lions). If they were back in the wild, they’d tend to keep and hunt within a territory and do so alone. Even feral and outdoor kitties will naturally choose to spend a large portion of their time exploring, wandering around, and defending their territory alone. These cats only rarely interact with other cats, and in fact avoid interactions as much as possible. And to that end, a cat will set up their territory with their resources (food, water, warmth, bed, you) and often won't appreciate new cats (who will be strangers to your current kitty) wandering in and encroaching on their turf. Furthermore, a new cat may not be overly keen on another cat already being present and dominating their "new territory". So, we are asking them to go against what is instinctual or natural for them. And with this in mind, we want to take everything very slow with lots of patience and perseverance to give them the best chance of getting on with one another.
Furthermore, I would say that unless you found another docile or laid back female, it likely best to avoid a second female cat. The reason is because cat society when housed in groups is a matriarchy. Therefore, we are more likely to see territorial disputes between female cats. So, if Meg is docile there is a high risk that a dominant or flamboyant second female cat would bully Meg (and try to drive her out of the territory).
As well, just to note, since this hypothetical new cat would be coming from an unknown background and has been in a rescue (so has a high possibility of carrying agents like cat flu), it would be ideal to consider getting Meg vaccinated at this stage. And of course, we'd want the new one to come in already having been vaccinated. The reason why it is important for Meg to be vaccinated is because this will ensure she has some immunity (which she likely has little due to being indoor only and never vaccinated) against those agents many rescues come in with and reduce her risk of catching anything or at least reduce severity for her.
Now onto the actual introduction. As you noted the first key is the patience element. The introduction should be gradual and cats can take weeks to months to years to adapt/accept one another fully. And just like some people (with certain workmates or roommates), some cats might never become friends. Some cats will just not gel, and get on with one another. So, while I don’t want to worry you, I do I want you to have open eyes (and be ready if the road to kitty introduction is bumpy).
Now the first step would be to keep them separated in different rooms and allow your established cat appreciate the presence of the new cat and get used to the idea of their being in the house (smelling them under the door, etc). From here, you can consider feeding all the cats by this door area so that they can continue to become comfortable in one another's presence and can associate one another with positive experiences like feeding time. This can also help reassure Meg that the new cat won't be stealing her resources.
If Meg is calm, comfortable and not stressed by smelling them or eating near by, then you can try supervised visitations at the door. If anyone react negatively at these wee meeting, you want to stop them meeting, close the door, and try in a few hours. Once everyone is calm about seeing one another, then you can let the the new cat out for supervised visits into your established cats territory. Of course, if the new cat try to bother your established cat (or Meg gets upset) then that is your cue to put the new cat away for an hour or so. Let everyone chill out before trying it again. Once the established kitty gets calmer about sharing a territory with the newbie, we can consider having them all out when you are home (though separate when you are out). The eventual aim is for all parties to become desensitized to their presence and thus tolerating their sharing their home/territory.
Further to all this, it is always a good idea to use a de-stressing agent during the early days of introduction to reduce the tension levels. To help with the general stress of introduction, there are tools we can use to help provide a general peaceful environment for our pets while they are adjusting. Often we will use Feliway (also known as Comfort Zone (http://www.petcomfortzone.com/cats/products.html) in the pet stores), which is a synthetic cat pheromone that helps to relieve stress and promote relaxation of the stressed cat. This can be used as a spray or a plug-in diffuser. Otherwise, there are nutritional supplements available OTC from the vets, like Kalmaid or Zylkene, which can help in these cases. The aim is to get everyone calm and open minded, instead of being on edge and turning to the defensive/offensive when in sight of one another.
Furthermore, if you find the new cat is overbearing and bullying Meg, then do consider providing Meg with a separate space to ensure they has a place that is still their territory. This can be a favorite room but should ideally have all their resources (ie food, water, litter pan, cat bed, etc). It will be a place that they can feel secure and should be a place that you keep off limits to the new cat.
Overall, introducing new cats is often a challenge but with time we often find that cats are able to at least form truces within the household. So, as long as your you can find a laid back/non-dominant/docile kitty (potentially male being the best option) that is open minded to sharing a house with Meg and you take introduction slowly with them, then you should be able to avoid the typical pitfalls and help establish a peaceful kitty household.
I hope this information is helpful.
If you need any additional information, do not hesitate to ask!
All the best,
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