I'm thinking two different kinds of spren go into Plate. The lesser spren of each Order is the majority of it, or at least the structure that the rest of it is based on, and making up the difference are the same kind of that the half-shard fabrials use to increase their solidity. It's probably the same spren that show up when Kaladin binds something (Connectionspren? Gravityspren? I don't remember which one they are), and they probably merge into one another, leading to new Platespren that are content as long as they're in their shape.