Migrators, scouts and perhaps even minelayers should be separate group.
Agreed, but then you run the risk of getting too many buttons on one little structure (which I wouldn't like). After all the Vasari already does, having some risk/randomness to the Dark Fleet Beacon is a nice catch. Can't have your pie and Pinkie too. Where would the special selection stop? It'd be like the Vasari version of "The Price is Right:"
"Jarun Migrator from Alpha Centauri? Come on downnnnnnnnn! You're the newest member on 'The Fleet is Right!
'"P.S.: If the Vasari have technology so advanced that they can hand-pick what ships to call for from the Dark Fleet that may be systems away, what does that say about the other two races?