I would be careful with bringing in a stray adult cat with kittens. That Mom is going to be very protective of her babys.
I have a 22lb Maine Coon Cat who is 14 yrs old. I got Sadie 3 yrs ago and Lillie 2 yrs ago. They DO NOT like him! Poor guy..All we have to do is mention his name and they jump at his face. God forbid if I pick him up! they jump up and bite his fur. I (they) are very lucky that he is so patient with them...he could really hurt them. I don't know why they do this...He was here long before they were. I had toyed with the idea of breeding Ragdolls but, because of the "love" of cats my furbabies have that doesn't look like it's going to ever happen...Once he is gone their won't be anymore cats in our home.