Hi my son has a trek marlin with post mounted front and rear and needs new cheap brakes so I found some Clarks m2 they say 180 f 160 r but have the old style adaptors can't remember the name the ones you have to shim, I presume I can't just stick them on without the adaptor, or if I did would they only work on 160 rotors thanks hope that makes sense

That's it, if you pop the brake directly onto the Post Mount frame and fork w/o those old IS mount adapters it'll fit 160mm rotors (make sure the bolts you use are long enough to get most of the way into the frame or fork posts, as well as aren't bottoming out in the post, before tightening). For a 180 front rotor you'd use an F180P, a front 180 PM adapter like this -

Cheers mate much appreciated