Ok, so am I allowed to switch my melee in the middle of a run? I don't see any rule against it but I wanted to make sure.

I don't see why you would want to?

Oh you probably want to poost until you get to the gate in gatehouse fruit speedrun, then switch to key

Pretty much.

Rules dont state otherwise, though it comes with slight time loss

Ya it does. Though I had an idea for gatehouse wherre u run out of spawn and then edge/prop poost of one of the weird arrow holding objects over the wall straight to the front gate, which would skip all the parkour. Spent 3 hours trying this, got very consistent and had the height on most jumps only to figure out there is an invisible barrier ontop of that wall.