I was a little confused about what you meant by volunteering at first. I thought you meant going to a shelter to help socialize the cats already there, but it seems like you're talking more about fostering?
I think cats could potentially benefit from being around other cats, but it's probably easier the younger they are. Your little guys are still young, and they already play with each other, so they might like spending time with other kitties.
Then again, they could hate it. They are cats, after all.
But there's only one way to find out! And you're right - it would definitely make things easier for any future kitties that might end up at your place.
I've wanted to foster for a long time, but Margaux was a bully who barely tolerated Celia, and now Celia is a skittish little old lady. If I foster a cat, it'll be as a trial run for adoption.
Your boys got chummy with each other right away, so they seem like very social kitties anyway. But they are still kittens...if you bring other kittens to your place, you could have mini tornadoes running around everywhere.