oscars and a few silver dollars would probably be ok in that size tank. tinfoil barbs grow twice the size of silver dollars so they'd do better in an even larger tank, imo.
with a jag, you might be able to get away with silver dollars, it's kind of hard to say. the one good thing is that silver dollars are very fast swimmers so they probably wouldn't get damaged too much but there's the chance they could get chased around all the time which would be pretty stressful for them.