You can keep other fish with oscars.. Most places will tell you it's possible with the right fish (as with most fish)
I would up the filtration. Preferably to somewhere in the neighborhood of 10x turnover per hour. A canister will likely be the way you will want to go.
People have also told you that you can't keep 8 red bellys in an 80g. You haven't said anything about this which leads people to assume you are ignoring them.
Read my other reply for my reply to the 2nd and 3rd paragraph.
I would up the filtration. Preferably to somewhere in the neighborhood of 10x turnover per hour. A canister will likely be the way you will want to go.
People have also told you that you can't keep 8 red bellys in an 80g. You haven't said anything about this which leads people to assume you are ignoring them.
Read my other reply for my reply to the 2nd and 3rd paragraph.