a 3'x1'x1' tank? ...thats probably why your oscars are fighting (small tank territory issues)..i would only attempt that with a biger tank..
i've tried with my Jar before (1 jar + 1 oscar) at those sizes in a fairly large tank and it didn't work out...the oscar actually pick on my Jar ...so back to the LFS went the oscar.
its been done before but, personally would recommend it..at least till your arrow is bigger.
It would be best if the aro is about twice as long as the oscars. Hope you realize that in about one year a 6" silver aro will grow to be 20 to 24" long.