Just what a 'turret' is is not explained in Book 2, nor does it way where the gunner needs to be (aside from in passing in the description of the 'Gunner Interact' program). I'd assume they'd be in the volume allocated to 'fire control', wherever that might be (there's nothing saying where it has to be in relation to where the 'turret' is placed).
All we know about turrets is that they go on 'hardpoints' that must be allocated during design and construction, that ones with more capacity cost more, and that they require a certain amount of interior volume for 'fire control'. Book 5 makes it clear that they require significant power, aside from sandcasters (and missiles, pre-errata). Interestingly, B5 removes the cost of a hardpoint and of turrets, and then makes the tonnage requirement be for the turret.
We can safely assume that hardpoints have power and data feeds sufficient to manage and power up to three lasers. Presumably that's what you pay KCr100 for.
IMO the lasers are the size they are because they fit into the volume that a missile (or sand) launcher plus a two-round magazine takes up or that launchers happen to have a two round magazine because that's how many you can fit (plus launcher) into the volume the standard laser requires. Presumably this happens to be the most workable size that lets you put three into a 'turret' mount.
As for the gunners, aside from controlling a turret, they also seem to be expected to act as loaders once the ready ammo is used up. As both sand and missiles mass 50kg each, taking about 5.5 minutes per missile to reload (three per turn) is probably reasonable. Note that B5 doesn't concern itself with such minor details, which is reasonable for military vessels that would probably have automatic reloaders and so on (but which should also pay for this in volume and credits, but don't).
Book 2 is fine for describing merchants, scouts, and armed para-military ships that are using what are essentially civilian weapons. It really doesn't handle warships at all, as it treats them as being civilian ships with more guns (and probably more acceleration and also probably bigger computers for more combat program capacity). But even if a warship can only have the same listed weapons, it should be able to fit bigger missile magazines with automatic loading and launchers with a higher rate of fire, and centralised fire control so one gunner can direct multiple turrets. Even if automatic loaders are out, rules for applying more people to the reload process should be in place (if nothing else it gives the marines something to do whilst waiting for a boarding action).
Book 5 on the other hand, abstracts too much - or at least doesn't bake in the extra expense military grade launchers, etc. should incur (heck, it doesn't even bump up the cost of turret weapons to cover the cost of the turret and hardpoint that it abstracted away).