1.The decoupling caps need to have low inductance, very low leakage current, and low microphonics.
2.The capacitors need to have tight tolerance (1%).
3.Unfortunately not, both +5V and -5V are also used for analogue circuits, in fact both +5V and -5V appear to be most critical. I had best results with approx. 3000uF (multiple caps in parallel) for power supply decoupling (TDA1541A).