damocles.
Re your questions about multiple weapons per turret. I have done that, and have seen it done on other designs posted on the web. Unfortunenatly I have no idea how to post a link even if I could remember where it was so I can only tell you of my experience at this.
Point one, In my opinion you will NEED Fire Fusion and Steel to do it.
Point two, you may have to "homegrow" rules about implementing them to your traveller universe.
FF&S has a design sequence for both lasers and spacecraft missile launchers. Therefore all you need to do is use these sequences to make laser's, missiles, and their launchers "small" enough to fit in a turret together. The TNE standard missile is seven cubic meters, thus with supporting infrastructure, ie the workstation for local control and the size of the launcher itself, you will not be able to fit more than two "standard" missiles, or one standard missile plus one smallish laser per 42 cubic meter turret.
The Bard pages at Downport.com have some great smaller missiles listed as "Solomani" missiles. Five of these fit in launchers in a turret without a local workstation therefore requiring an MFD on the Bridge.
With mixed weaponry though, how "you" determine what the gunner is controlling will be up to you.
As for adding sandcasters here is where it gets a little bit trickier. You'll need to reverse engineer the example turret. By that I mean determine the space required by the various components at the various Tech levels. Obviously the workstations will allways only take 7 cubic meters. From that check the size of each individual sand cannister and how many fit into a turret at each TL.(cannister size is in FF&S) From that you can determine the required size of the sandcaster "launch & manipulate" mechanism. To then add a weapon you delete sand cannisters as appropriate to the volume of the weapon you are adding. If you retain or delete the local workstations is a design choice for your own preferrence, however the sandcaster will need a workstation somewhere.
Again you will need to devise an IYTU ruling on which can be controlled per combat turn? one or the other or maybe, at + 1 diff mod per task, both.
IMTU I have designed turrets that have:
a sandcaster plus one missile, but the missile (or sensor probe) can only be controlled from elsewhere on the ship and the caster only has about 12 cannisters as opposed to the comparable TL's 24.
a small short range hi ROF laser and a missile tube with the same restrictions.
a twin laser turret with MFD as part of the local workstation built right into the turret.
and
a tripple laser Barbette also with built in MFD
My twin laser and tripple laser mounts only mount one beam pointer however so IMTU all lasers strike the same target location if a hit is rolled due to the single beam pointer being the on mount fire controll ie no multiple targets or target locations per target per combat turn (each focal array generates a seperate hit ofcourse)
arrrgh if I add anymore I'll be rambling
Hope it helps.