Freaking out over potential Felv risk!

I did a very stupid thing, and now I am massively freaking out, and I need some advice from all of you cat pros. 2 weeks ago I brought in my little outdoor stray that I have been feeding for like two years. I had him neutered and flea/ticked/tape worm treated, but that was pretty much it considering I thought he would be going right back outside after he had healed but....two weeks later, he is still here. He was never Felv tested, and my 3 indoor cats were never vaccinated because they don't go outside, and I never dreamed I would bring in another cat. I have been keeping my outdoor cat in a separate room for the most part, but have been letting my boys in just to see how they would react, but for the most part they have kept their distance. They've always been supervised when they're together, and have not touched one another.  But just today, stupidly I let my outdoor cat out of his room, and while he was roaming around the rest of my basement, I'm pretty sure he used my boys litter box, and then one or more of my cats used it afterwards. I am terrified that if my outdoor cat has Felv, that I may have just inadvertently infected my own boys. I'm calling the vet first thing tomorrow to have him tested ASAP and he is NOT allowed out of his room until I know,  I am so worried. In your experience, do you think it can happen that easily? With just one interaction like that? Any advice would be appreciated!

If your residents have had the vaccine, they should be okay.  Get your newbie to the vet for the test and put your mind at ease.

Have you seen this Cornell article?

Feline Leukemia Virus

You don't know whether the new boy is positive, and even if he is, the chances of transmission through one-time use of the same litter box seem pretty low. There's no reason to panic; get him tested and go from there.

What @jcat said.  If the cats haven't interacted physically, chances of FeLV transmission are really low.

Try not to freak out, do get the new little one tested, and let us know! 

FeLV can't be passed through fecal/urine matter, only sputum. as long as they don't eat from the same bowls and groom each other constantly they will be ok! they have to be exposed to the virus in large amounts before they actually contract it. I have a positive kitty and negative kitty and have done a lot of research on it. if you are worried... buy fresh litter and clean the box really good, get your boys tested for the virus so you will know for sure. also older cats don't get it as quickly as younger (not sure of the age of your indoor kitties). good luck !!!