Ok, I checked the comp enhancement file and there isn't a slot for normal mounts, so I made one

This should clear up any AI issues, though I don't think light mounts on destroyers would have been that big a problem.
Long Name := Light Ship Mount
Short Name := Light Mount
Description := Smaller sized weapon mount which decreases damage from the weapon by 1/2 times. Requires a vehicle size of at least 150kT. Can only be used on Direct Fire weapons.
Code := S
Cost Percent := 50
Tonnage Percent := 50
Tonnage Structure Percent := 50
Damage Percent := 50
Supply Percent := 50
Range Modifier := 0
Weapon To Hit Modifier := 0
Vehicle Size Minimum := 150
Weapon Type Requirement := Direct Fire
Vehicle Type := Ship
Long Name := Standard Ship Mount
Short Name := Standard Mount
Description := Standard sized weapon mount which does normal damage. Requires a vehicle size of at least 300kT. Can only be used on Direct Fire weapons.
Code := N
Cost Percent := 100
Tonnage Percent := 100
Tonnage Structure Percent := 100
Damage Percent := 100
Supply Percent := 100
Range Modifier := 0
Weapon To Hit Modifier := 0
Vehicle Size Minimum := 300
Weapon Type Requirement := Direct Fire
Vehicle Type := Ship
There ya go folks, if you don't want the AI's destroyers running around with light mounts, this should clear up that issue =)Have fun.