Always go first means instead of Karryn going last in the round, she goes first. So with that particular skill, she can initiate a blowjob session before an enemy grabs an open slot. However, since her blowing someone doesn't mean that her other slots are taken, someone can initiate sex with one of those slots even if she is blowing someone. The only things that are blocked are those that require her mouth slot to be open.
Most enemies look for certain slots to be open. Rogues are known to go for the rimjobs for example, while nerds will typically go for the footjobs. When initiating a scene they will always pick their favorite pose, and if they can't (because Karryn's desire isn't high enough), simply pet/spank/insert a toy/etc and let someone else take their turn. If someone else started a scene already, they can join the scene as long as a slot is open and Karryn's desire is high enough for that particular act. For example if a rogue started a rimjob, a guard might join anally if Karryn's butt desire is high enough and the guard's horny enough.
There's a lot of code covering this, and I'm honestly not sure exactly how to edit it properly, but I would take a look at RemtairyEnemySex.js if you want to get into it.
