Depends on where you are going to put it, as spouses tend to object to big plastic barrels plonked in the house (fortunately my long suffering wife is very understanding). I believe in Australia you are spoilt for choice as far as water catchment containers. Is the ageing butt going to be on the same floor of the display tank? You are going to be better off if you can rig up some sort of permanent hose to empty the tank and another (with a pump) to get the water from the ageing barrel back into the tank. In the barrel to agitate the water you can 7se either an air-stone/pump or a wavemaker (I use a SunSun JVP 100).
As for companion fish the most tried and tested are rummy-nose and cardinal tetras (but I would also consider lemon tetras) and Corydoras sterbai