I prefer group_x to acecommunity. If we want to detail the community aspect of the comm, perhaps something like ace_umbrella might work better. I don't know why, but that seems a little more... inclusive than ace_community, as well as capturing some of the fun of group_x.

Also, at least within the site, I don't think that having "ace" in the username will make it any more easily searchable. I know that I personally rarely if ever use the username search because it doesn't search partial strings. So the username really doesn't matter in terms of finding through searches, because the person would have to already be looking for "acecommunity" or "group_x" first. And then in terms of searching by interest, there's no real advantage to having "Ace" in the name. Maybe for off-site searches, but those engines tend to index the content of individual posts, not just the community name.