I'm a bit confused with this let's say my arceus or kekleon are poisoned then in kekleons case is hit by a steel move and in arceus case trick gives him steel plate.
What happens now?
We have 2 steel Pokemon poisoned.
Would the poisoning go away or would they not change type?
Well first of all you can not use Trick, Knock-off, Switcharoo, or any item removing move on a Pokemon with Multi-type(Arceus). As for Kecleon it will still be poisoned.