It does not matter if you get a female or a male cat, the important thing is how you'll get your other two cats to like the newcomer kitty. It is really normal for cats to hate each other at first, you know, trust issues. But I know that in the long run, they'll get along well. :)
Your rescue cat is showing signs of rebellion. He is missing the litter box on purpose. It can also be because he wants to get your attention, how often do you pet him? I suggest you bring him to the vet, he might provide you with remedies.
P.s
Please, don't get rid of him. He was once a...
Here are the telling signs you should watch out for to determine if your cat is pregnant:
1. Morning sickness.
2. Larger Nipples (check)
3. She looks a little swayback with round bulging tummy. (Check)
2 out of 3. :) Congratulations. Sammy the cat is pregnant :)
She might be experiencing a urinary tract infection. You can refer here to determine if your feline is showing symptoms of UTI. http://jacarandaanimalhospital.net/five-telling-signs-causes-uti-cats-dogs.html/. Hope she gets better soon. :)
I hope he gets better soon. You can also get him to other vets for a second opinion. Maybe he's just experiencing some digestive distress. How is he doing now? Any update?