Yeah got a nice blue tier 2 cannons being tops in something (cant remember what it was) and scratched my head for a few minutes wondering why I couldnt equip them on my cruiser. At least I can sell them for 5k in my replicator.
Cannons can be equipped on Escorts, Birds of Prey, and Battlecruisers. Battlecruisers are ships that the KDF gets from tier 3 to tier5. They are the KDF equivalent to Federation cruisers that sacrifice EHP (roughly 8-10%) for a much better turn rate and the ability to equip cannons.
The only cannons that can be equipped on ships not listed in the above classes are Mk1 and Mk2 cannons (which have no restrictions), and turrets.