Hi
I know how you feel. I took in a stray to, but I don't have any other cats(yet!) I was told that if I was even considering having another cat that I would want to test the stray for FIV and FeLV. You never know what the poor stray has been exposed to and you definetely don't want to infect your other kitties.

My vet told me that FIV and FeLV was basically only transmitted through bodily fluids. So, cats sharing the same litter box, food and water bowls can infect each other. But, if you can keep the stray kitty completely separate from your other cats, I see no reason why the kitty can't stay inside until you can take kitty to the vet.

And as far as your other kitty adjusting, well I can only tell you what worked for my ferrets...Rory, the oldest, became quite stressed when I brought in a newcomer. I gave him extra playtime with just me, lavished attention on him so he felt felt like top ferret, and gave him special treats and out time where he is usually never allowed. That special treatment and supervised playtime with the new guy on neutral territory got Rory to accept the newcomer. Just my two cents Good Luck!