Smashed my rear mech last weekend. got a new one from Merlin. fitted it and noticed chain is stretched.(I did the pull it away from front chain ring test). Do i need to change rear cassette as well. and if so what about font chainrings. All kit is XT.

A new mech on a worn drivetrain shouldn’t cause any issues.

If you are worried you could always swap the jockey wheels between mechs, but make sure you use thread lock to stop them loosening. But again, don’t think it will make any difference.