Hi Lisa -
I have more experience w/ FIV, although I've also had Leuk+ kitties.
I have 2 adult FIV cats. One came to me as starving kitten (Stanley); the other was a neighborhood stray that showed up often w/ large patches of fur missing, as well as a chunk of his ear (looks like pinking shears!) (Moochie Pooch). Moochie is now approx 9 years old. Other than needing multiple (3) rounds of AB when he showed up (around age 2) with acid burns on all four feet, and more recently some peeing issues (now being treated w/ ____), he is doing great. Stanley is a big lump of love. He's one of the 20 pounders in the house, and is happy to have a safe dry warm/cool home, a soft couch to sleep on, and regular food (he was a starving when he arrived about 4 years ago). Neither have issues, and both interact w/ my other cats.
When I rescued the ~2 week old "first five" after their mom was hit by a car, I bottle fed them. Two had a reaction to the formula, and when I took them in, they tested them. Their paws were barely the size of a child's pinkie, and they had a hard time getting blood. The tests came back FIV+ and the vet (who I never saw again!) recommended destroying the entire litter. About 2 -3 months later, I had them retested, and they came back negative. I would imagine that 2-3 weeks is not long enough between tests. I would probably wait at least 2 months.
One of Moochie's litter mates I trapped once, but had to let go, as animal control wouldn't allow me to have him neutered... said he was now "property of the county", but I could go and adopt him! (he'd been eating in my garage for months). When I was FINALLY able to trap him again, he tested Leukemia+. Vet said to either bring inside, or PTS. He came inside, and had his own room. He only lived about a year.
And most recently, I adopted a Humane Society rescue (Hope) who I had taken for treatment for an eye ulcer. She was very fragile, but after a few months had finally begun to thrive. Unfortunately, she took a turn about 1.5 weeks ago. Fluid in her lungs and abdomen, and major difficulties breathing - she had to have oxygen just to get through the x-rays, and the doc said the prognosis doing everything possible, might get me 3 weeks. So after spending only 6 months or so with me, I had her PTS last week.
That being said, would I do it again? Absolutely. The time she was here, she was one of the sweetest, most loving kitties I've ever known.
I did let her interact w/ a couple kitties, including Moochie Pooch. None actively groomed with her, and she always had a separate food bowl and food was taken up after each feeding. Otherwise, she was treated like one of the gang. And there have been no problems with the others.
Caring for a "special needs" kitty is very demanding sometimes, but as you are aware, the rewards are amazing. Good luck in whatever you decide.
Lu-Ann