With leopard geckos, as long as there aren't multiple males, it isn't a bad idea to house them together. I have a display tank at my house that i have 1.2 adults in and I have never had a problem with any of them. You should def quarantine any new ones before you put them in with existing ones. When you do put them together I would keep an eye on them at first to make sure there is no bullying going on as i have heard of some leos that dont play well with others.