If the Slave liner is to be sold with Slave Liner Turrets, and there's no ruling that they may not be traded out for Transport Turrets, then there should be some reason or another that they aren't traded out for Transport Turrets. Like, they hit harder, or they look totally awesome (which they don't, right now), or they provide pseudo-Battleship functionality to the ship (i.e. get close to me and you die- which would be lower range but far better damage). Something like that. But right now, there is no reason not to swap them out other than cost... which is pretty marginal, actually, especially considering that the ship costs over 100 million credits to begin with.
In summary, solutions to our conundrum are-
Make a ruling saying that the Slave Liner cannot use Transport Turrets
Slave Liner Turrets are given a niche of some sort- such as short range devastation, etc.
Slave Liner Turrets are made to look/sound so impossibly cool that it's okay that they're inferior
Slave Liner Turrets are made to be "simply better" than Transport Turrets, possibly with a ruling that Slave Liner Turrets cannot be mounted on things that aren't Slave Liners (or, maybe, restricted to Slaver ID)
I like the second option, although I would settle for the third. Thoughts?