I think one may be pushing it for a 30g, its a little tight.
I have two in my 55g. They are always chumming out together. They do this wierd dance. It looks like they're sniffing out each others rear end, kind of like what dogs do when they first meet each other. They also play follow the leader a lot. Its to my knowledge that they are more active in small groups (like 3). Well, not to say one won't be active, but a small group, given that they are comfortable in their tank, will socialize more.