Will adding a female cat for breeding cause problems with my neutered males?

It's a little difficult to say. The female must be "induced" to ovulate, and without some kind of penetration (which I doubt your neutered boy was capable of) it seems a bit unlikely. However I would assume that being housed with an intact male should cause her to ovulate, once she allows him to mount.

Neutering male cats is one of the most effective ways of limiting inter-cat aggression. There are no guarantees, but castration is definitely the first and best thing worth trying. If you are worried about the two cats hurting one another, I would keep them separated for the time being. Even after the castration there will still be some residual hormones left in the body for a short while. I would consider keeping the two separated for at least a week or two post-operatively before reintroduction. Hope this makes sense.

I have never heard of one cat killing another over mating issues, so you shouldn't need to worry about this :) However, I'm sure it is annoying for your female cat to have him constantly trying to mount her, and this can certainly lead to developing aggression issues between the cats over time. I would try to get your male kitty neutered as soon as possible, to help prevent this from continuing to happen. In the meantime, if he won't leave her alone, you may need to keep them separated until after he is neutered.

If the two females cats are not spayed I suggest you do that as soon as possible. That could sure be the reason for the tension. Not only that but if the females are not spayed and are going outside they will get pregnant guaranteed as soon as they go into heat. Usually the first heat cycle is around 6-7 months (so now). I also suggest you try some Feliway products in your home. These are basically "happy cat pheromones" that help to de-stress and relax cats. It works very well. Here is a link to their website. You can buy their products at any large pet store or online. https://www.feliway.com/us#redirected Hope this helps. Best wishes.

Introducing a new cat to the household can be quite disruptive. You will likely have to go back to the beginning, and give them time apart for several weeks. This will mean keeping the new male cat in a separate part of the house, where he can't be around your female cat. You don't even want them to see each other at this point, but it is ok if they are able to smell each other under the door. After a few weeks of this, try letting them see each other, but not in the same room. Finally, you can let them be in the same room, but make sure they are supervised and make sure that each cat has a way to get away from the other. I would also suggest that there are more than enough cats in the world - too many, in fact - and I recommend you considering getting both of them altered, instead of trying to force them to breed.

I would not recommend bringing in a stranger cat to mate with your male. Not only will it cause stress for Habia who has a litter of kittens to look after, but you could be creating another litter of kittens with this mating, which is not responsible breeding unless you are planning to look after this female and her kittens as well. In addition, allowing him to mate this time will not stop the calling, he will continue to call because this behavior is hormone driven. The only way to stop the calling is to have your male neutered, which I would recommend.

I'm sorry you're having this situation!  It increases the stress for everyone when inter-cat relationships are aggressive.   While uncommon in neutered males, sexual aggression is still possible. Here is an interesting article from Tufts university regarding this issue.     I haven't personally run into this in my practice, but I have heard of Boar Mate spray being recommended.  Sometimes anti-anxiety medication such as fluoxetine can also be helpful when given to the aggressive cat.   Talk to your veterinarian about specific recommendations for use. http://www.tuftscatnip.com/issues/22_1/feature/Understanding-Feline-Sexual-Aggression-222-1.html