In my, very large, family experience, I say let them do as they wish! Those that don’t want to mingle will gravitate into single-gender groups and members of the opposite gender will leave them alone. Those that are more comfortable getting to know their cousins will do so. (Those are usually the vocal, opinionated ones who tend to dominate the conversation. They need as large an audience as they can get.:) )
Most folks that I know look on first cousins in terms of the “ew” factor and wouldn’t seriously consider a shidduch with one of them.