I've always just taken the newbie in for the test right away. The vet techs do it, no appt needed where I go. I don't even want ME to get attached if the cat can't stay.

FIV + the cat can stay and mix with the other cats. FLV+ is a bit trickier and many people feel they should be segregated or only mix with other FLV+ cats. I have one FIV+ in my mix.