Okay, firstly almost all of the Prochilodus species grow big, 300mm/12" type size. They grow quite quickly given adequate space, but take several years to reach full size - like most larger fish, they grow very rapidly to about 60-70% size then slow down.
I've never kept taeniurus, but have kept insignis which is almost identical. They are peaceful enough when small, but can get very territorial as they get larger, and being front toothed, can do a bit of damage if the fancy takes them.
They are lawnmowers in a planted tank - real bad news.
I think they would be a little boistrous for a Discus tank.
I've never seen them in the Negro type water systems. They are more common in the solimões water regime, thus may not tolerate the higher temperature.