I haven't been able to confirm what Grand Nagus said. If he's proven incorrect however, that's probably because a) Cryptic hasn't gotten yet to fix this; or b) it was done to circumvent certain exploits.
I'm kind of siding with Grand Nagus. That's what I did for my end fight (after a little revision). I even went so far as to reskin the ship so it's nor exactly randomized.
But yes, to echo what Grand Nagus said, the bottom (lowest number) of the drop down list for the NPC group in your placed grouping seems to be the first of the group to spawn. I assume the rest of the list would spawn according to level and the player's group size, likely in that bottom-to-top order (in the event like me where you renamed the actors without numbers). This could probably bear a bit more experimenting to really make sure.