I just added a small Oscar in with one of my bigger oscars who's about 8 inches or so out of an emergency situation. He got picked on a little bit but he seems to be fine. I wouldn't recommend doing so unless in dire circumstances though cause there's no guarantee. They may not eat them but they will get bullied pretty badly and the convict will nip the crap out of them. If you prize the pair, don't do it. Just let them grow out a bit so they can handle the damage.
Yep. Don't be surprised if your Oscar eats the smaller one. They'll take a whack at any fish that can partially fit in their mouth if they feel like it. My O tried to digest my FM when he could only fit half the FM in his mouth.
Unless you are feeding food 24/7, it would be almost impossible to keep them well fed all the time.
Oscars get hungry quickly! Their stomach are like bottomless pit....