I don't think its necessarily the tang & the damsel you have to worry about (although they are still a threat), but its the other clownfish itself. You should try to purchase one that is smaller than the one you already have now, and add him with the lights off.
Obviously, though...this is a "maybe, maybe not" question
. I haven't had any experience personally, but I am sure others will be along to help.
BTW, its suggested that scopas be housed in a minimum of 70gal, so that may also add to some aggression he may show.