I have a rainbow shark, he is not a problem at all, he is maybe 1/2inch shy of fully grown, and lives well with all the tetra he will attempt to chase a few pentazonas off 'his' cucumber slice, but thats fair enough........one barb is fine with him, a few and he just knows they are going to eat too much.
I try to put the food in late in the evening as he is (in theory) more likely to continue eating it through the night, but when ever I see him at night he has found his place on the bottom (strangely only slightly under a large material plant leaf than in a hidy hole) and seems settled there until morning.
He seems happy, patrols the tank, he is king of all he sees and he seems to know it (there are 6 'large' tetra and they seem to not notice him, and 14 'small fish' little tetra and small barbs so no competion for him there) he doesnt seem to feel he has to prove it!