No you can't because they grow astonishly quickly, the oscar will hit a foot and the pacu about 3 feet I think. They will grow quickly, eat the others and then die of ill health. You need at least a 55 for the oscar big, big , big for the pacu. Don't even bother buying the fish if you haven't got those big tanks as they'll get big real quick - I had a tilapia grow from 3 to 10 inches in about 6 months, and these fish might do the same.
Go and look at some full grown examples of these fish in a good shop - not petco or whatever, and decide for yourself if they'll go in a 20.
Realistically if you want to keep big, character, aggressive fish you need a much, much larger tank than a 20. You could keep , maybe some lake tangs, but only a pair of brichardi for example, or a trio of peacocks. But you will ahve to choose very carefully.