Hey, guy, I take small gay groups on mainstream cruises about 6x a year, so I'd say you should book that cruise. Don't worry about cliques--the guys in the group enjoy meeting other gay guys, or else they wouldn't have joined a group. When our groups sail, we always bump into a few other "family members," and they invariably end up joining us for excursions, dancing, etc. In other words, I think you'll have a great time and fit right in. My only question is: Why not just book into the group? Is it closed? Is it more expensive? The group will probably have some special amenities, excursions, etc, so maybe you should think about becoming a part.
Have fun!