Weren't the Mogami-class initially Light Cruisers by initial design?
Only due to the guns they carried. Treaty limits after the London Treaty had light cruisers with 6.1 inch guns or less, while heavy cruisers could go up to 8 inch guns. There was a limit imposed on the number of heavy cruiser tonnage a nation could have, so countries produced more "light cruisers" in the 1930s. Cruisers that were only light because they shipped 6 inch guns, but were all at the upper cruiser limit of 10,000 tons each. While the British didn't do this so much, the Americans and Japanese built several "light cruisers" with fifteen 6 inch guns, including the Mogami-class. They were really over the treaty limit of 10,000 tons, and the Americans knew that, but the American ships were also too heavy. The Mogami-class, unlike the American Brooklyn-class, were designed with the intention of replacing the 6.1 inch triple batteries with twin 8 inch batteries once the treaty was no more.
Yamato and her sister ships were to get several of the removed 6.1 inch triple turrets from the Mogami-class ships that had their light cruiser turrets removed to become heavy cruisers. Each Yamato got four turrets of the five that were on each Mogami. Later they removed two of these from the Yamatos to install a LOT of AA batteries.
It's experimental armament light cruiser Suzuya!The data is all fine too, yeah?Tell me your impressions of the equipment later, okay!