dissaccociatives are different from psychedelics like shrooms and lsd, its a different drug class containg dxm, pcp, ketamine. i would like to think of hallucienogiens as being life while dissociatives are death, both are important experiences but quite different. and to fully get dissocicative effects off dxm you have to know what dose to take cause small doses dont really give credit to the substance, id say take like a 1000mg (1 8ounce robo, and 1 4ounce) would do you nice for a good dissociative experience.
but back to the original question posed, what drug should you do first, id say shrooms before acid, but between those and dxm i say its your choice cause there markedly different experiences,