I think Calamity Mod Low% (Killing SCal with as fewest bosses killed up to that point) would be a cool addition to the leaderboard. It would push change the route of the game by a lot since Calamity makes a lot of the bosses optional.
Typically we only add categories that will actually get submissions and this one would probably not get any. If you do a run of this and post it or see somebody who has done a full speedrun of it we may consider adding it.
The problem, I believe, is that the a normal Scal run within the current meta already kills very close to the minimum amount of bosses.
In its current state, the leaderboards only accommodate runs with the Calamity mod. But this doesn't have to be the case. Calamity fits nicely into the level leaderboard layout, but has also warranted a subcategory that applies exclusively to that mod.
Categories for other mods can instead be made