Are your earlier oneshots about Ren and the Red King canon for Redstone and Sculk?

Yes and no.

Yes in the sense that they happened, no in the sense that they're not timeline applicable. I imagine the Ren and the Red King shorts happened before the events of Redstone and Skulk, the Red King and Ren have had their resolution -- whatever that entails -- and the Red King has mellowed out a bit since then. He's no longer new, hellfire and vengeance and righteous anger at the necessity of his creation. He's figured out who he wants to be in hels, and is being that thing.