It is wise to have a bit of a quarantine period, but if he’s coming from a shelter, I assume he’s had a vet check and perhaps been there for a while before he was adopted out? I found my 2nd bun outside so I had a 30 day quarantine, but that was because we have myxomatosis here and I had to make sure he was healthy + have him vaccinated before he could meet my first bun. I’d not assume it’d be the same with a shelter/rescue bun. If he had mites or worms or something when he came to the shelter, I think the shelter would have had him treated for that? You could perhaps wait a week? If he has sth, like some intestinal parasite or fur mites, the stress of being adopted into a new home could surface, but it’d probably do so within a week (swith symptoms like diarrhea/really loose stools or dandruff with hairloss) and get him treated before you let him meet your other buns.
Sometimes snuffles are caused by bacteria (pasteurella) that are commonly found in most bunnies’ nasal cavities. Some buns get the snuffles from it, but the majority of rabbits just have the bacteria and never fall ill from them. It’s not known why some buns get sick and others don’t. But dusty hay is a really common cause of sneezing and snuffles-like symptoms too, so that could very well have been it. It’s hard to tell after the fact, obviously.
I wish you the best of luck and hope to hear more about how you get on!!!