The few times I combined GABAergics with DXM I found it pleasant in low doses. In higher doses it gives you the spins pretty bad and isn't so cool, it can make you very dizzy and unable to function.
It will not cancel the DXM, it will make it significantly stronger in some aspects, but also may dull some of the psychedelic effects. Potentially a dangerous combo.
.5mg ativan isn't a high dose, but definitely enough to feel it.
As a general rule, it's a good idea to try a new drug by itself, before adding it to a combo of others, so you know what to expect.