I don't remember which of your kitties is FIV+, but the highest risk category are unneutered males (an unspayed females), fighting over territory or a female, or a female getting bitten during mating or in a territory fight. Once they're neutered/spayed, the odds fall off the map - especially if they're healthy - that they'll contract FIV. It can happen, but they don't know how it does, because it's rare.
All the vets we work with have clients that have FIV+ cats and FIV- cats living together, and they haven't seen one of the FIV- kitties contract the FIV. And once inside full time, even the FIV kitties tend to do well, because if they're not exposed to things that could make them sick, then the immune system doesn't have to fight illness to which they may be exposed. Most FIV+ kitties die of something other than the virus itself.
FIV+ cats do tend to have problems with their teeth, and that can be a source of illness or concern, so regular dental cleanings (and probably teeth extractions) are really important when caring for them. We just bought the Biotene stuff to put on Charlie's gums twice a day, and we add food grade hydrogen peroxide (1 1/2 teaspoons to a gallon of distilled water) to their drinking water (also helps fight gum disease). It's good for all the cats (and us, too!).
All the vets we work with have clients that have FIV+ cats and FIV- cats living together, and they haven't seen one of the FIV- kitties contract the FIV. And once inside full time, even the FIV kitties tend to do well, because if they're not exposed to things that could make them sick, then the immune system doesn't have to fight illness to which they may be exposed. Most FIV+ kitties die of something other than the virus itself.
FIV+ cats do tend to have problems with their teeth, and that can be a source of illness or concern, so regular dental cleanings (and probably teeth extractions) are really important when caring for them. We just bought the Biotene stuff to put on Charlie's gums twice a day, and we add food grade hydrogen peroxide (1 1/2 teaspoons to a gallon of distilled water) to their drinking water (also helps fight gum disease). It's good for all the cats (and us, too!).
).
Well, we agree with you. Charlie stays. 
), and an EXTREMELY cat-friendly cat. That said - he is the baby. He's the biggest cuz he's fat, but he's the youngest (probably the same age as Charlie, actually) of the previously existing group (by two years). I am positive he's jealous. Tuxedo is unquestionably alpha kitty these days (though Lazlo likes to pretend he still is, and sometimes Tuxie lets him think he's taking the reigns LOL). Billy's just... laying some ground rules, I think, and letting Charlie know he's not happy about sharing anything with him. Another jealousy factor - Charlie is on z/d because of his allergies (and he does seem to be itching less!!!!), and I think it's really bugging Billly that Charlie gets special food (he can smell it's different). I'm pretty sure he also equates Charlie with no free feeding: we tried to stop free feeding, but after the first few days, they figured out it was permanent and really let us know they were NOT happy about the situation - so we went to measured free feeding. It was going pretty well - but with the two different diets, unless we move ALL the cats to z/d, we can't have any food down when Charlie's here. Billy is fat because he LOVES food - so he may well equate Charlie with the lack of access to food all the time. 
. It amazes me how the hormones were hindering Charlie's personality; he is a lucky litle boy and was guided to the right humans.
I am not going to be surprised to see them curled up together soon. 
