You made a valid point, so I investigated how different weapons look ingame. I used Splendid Camera for close up looks and the results are:
MX family
1 long upper rail
1 long lower rail
2 short side rails (1 on each side)
Katiba family
1 medium length upper rail
2 short side rails (1 on each side)
no lower rail - in this case having a separate variant with GL, instead of GL attachment, makes sense
MK200
2 short upper rails
2 short side rails (1 on each side)
1 short lower rail
EBR
1 long upper rail
2 short side rails (1 on each side)
1 short lower rail, aft of which is bolted a home-made foregrip assembly
TRG
1 short upper rail
1 medium length side rail (on the right)
no lower rail - see Katiba comment
Having a QD suppressor or flash hider, is an open question for any of the rifles above and for gameplay's sake, I'd say let's allow for them, like it is now.
Now for the pistols:
P07
a threaded and protruding muzzle end for suppressor installation
a short dovetail-slide mount under the barrel
ROOK-40
nothing, although there might be a possibility to attach a laser or a small flashlight with some trigger guard adapter
In my proposition, there are only 4 rails + muzzle end, exactly like the MX model has. "Magazine" is not really an attachment point. I have separated long upper and lower rails into two slots each, in order to accomodate some combinations involving two separate attachments on one long rail, as I've described them above. Also, having a defined attachment points and point types, instead of freestyle rails, will make weapon and mod makers' lives easier.
I know that optics can be used on pistols. But given Arma's reality, I just don't see people sniping anyone from a pistol. Even if that would be really wanted, after examining P07 model, there is only one possibility to mount a sight there:
Which would make use of the current "Pistol lower" slot anyway.