[mayegelt]

The assassins mini-dex formations are either 1 of each assassin or just plain 1 assassin of your choice: it works just fine as the Culexus. (And there's nothing stopping you from taking multiple "1 assassin" detachments, so you could even field duplicates or a couple but not all if you don't want to take every single one of them)

Yep, nothing stopping you from taking more than 1 detachment of a single assassin, unless you are in a game that limits the number of detachments you are allowed to bring (most tourneys only allow 2 detachments max).