Its a trial and error situation...it may work, it may not. I put my 11" aro in with my 12" tropical gar, and he bit at her more then once, taking her entire tail and a few side scales. I put her back in her previous tank and she healed right up. I won't be trying this again for quite a long time until she is much bigger.
So, if you must try it, keep the gar well fed and def. keep an eye on them.