How active are your two cats? When I brought Saedie home, my other cat Millie HATED her. She hissed and growled but thankfully never attacked. Millie is extremely laid back. She rarely plays with toys but there isn't enough hours in the day for her to cuddle with people. Since Saedie is a kitten, she expects to play with whoever, whenever she wants. Millie didn't really like that..
Millie tolerates Saedie now. She will eat next to Saedie when I feed them, but to this day, Millie will not use the same litter box as Saedie. If Saedie decides to use Millie's litter box, Millie uses the plants instead..
I didn't really introduce them the "right" way, I kind of just brought Saedie home. I don't know much about introducing cats, if you don't know either I am sure there are plenty of other people here that can help you.
If you decide to get a kitten, sometimes it will be better to get two, just so they can play with each other instead of disrupting the other cats. Since you already have two, you might not want to have four.
I agree, make sure you get them tested. Since they are outside, you don't want to risk infecting your other cats.
Make sure you let us know what you decide.