Start capacitors are only used on single phase AC induction motors to provide a phase shift to the start winding to get the motor started. Both DC and 3-phase AC induction motors are inherently self starting and need no capacitors to run. Capacitors are also used to correct power factor in 3-phase power lines.
The capacitors on our DC model motors are strictly for RFI supression. Start capacitors are huge, hundreds of microfarads. RFI suppression capacitors are tiny, a few picofarads. I don't believe that the exact value is critical. It just needs to provide low impedence path to ground for RFI. A ferrite ring around a conductor can also act as a choke to block RFI.