One fire-control is enough to guide an unlimited number of Gauss guns or turrets as they can target multiple salvos. Although one gun or turret is needed at minimum per salvo... this make smaller Gauss guns more effective in general as they overkill incoming salvos much less then the larger Gauss cannons do.
On ships the most effective Gauss are the 17% reduced size one as it has 1 HTK so use that in a single turret or a a few mounted on a fighter hull. Larger guns might be somewhat better if you face huge salvos but their inefficiency against smaller salvos is so bad you generally don't want them outside role-playing reasons.
If you intend your PD fighters to do multiple jobs and their fleet PD role is one of them then build them, but you still should have dedicated fleet Gauss turrets as they are much cheaper and need far less fire-controls when mounted on capital ships or escorts.
So it seems that you are advocating for small single gauss turrets with reduced accuracy guns as more cost effective than larger, higher accuracy guns mounted on fighters because the higher accuracy guns find tend to overkill their targets and effectively waste their accurate shots. So even though the lower accuracy guns hit their targets less often on a per gun basis, because it is practical to field more of the low accuracy guns it becomes more effective to utilize several smaller, low accuracy guns than a single large, high accuracy gun. In general terms anyway, I'm sure there is a closet case for the large accurate gun, possibly as part of a more comprehensive and layered anti missile defensive system.
I'm not quite ready to abandon the PD gauss fighter concept just yet however, even though it seems that conventional wisdom is that gauss turrets are superior on a (mineral?) cost basis.
From a research standpoint, I am currently limited more severely by my BFC tracking speed than my engine power. For example, I can build a 20k km/s gauss cannon fighter but I can only build a 16k km/s BFC to target it. One obvious conclusion is that I neglected BFC tracking speed too much and now I am paying the price for my poor strategic research decision, so I will consider that for my next campaign. However for now, I'm stuck with this situation so it occupies my thoughts.
Perhaps I can build another fighter with a better BFC that can spot targets for the gauss fighters? Perhaps then I could improve my tracking speed sufficiently to allow my fighters to benefit from improved engines. Of course then I would also need to reevaluate the merits of a larger escort ship with a turret.
Decisions decisions...