you can spin the motor up, then connect it to the inverter. either connecting the inverter to a stopped motor, or connecting the inverter to an over sped hydro, will result in close to locked rotor amps in either case.
Hot plugging the hydro motor in with capacitors attached is a problem. the peak currents can reach insane levels and will either trip the inverter or potentially cause a high voltage spike when the inverter trips off.
Capacitors at the motor can cancel out the inductive reactance of the motor, reducing the line amps between the motor and the inverter, thus reducing copper losses in your line. how long is it?
Adding capacitors to the other phases can reduce the reactance of the motor as well and offset internally the current needed to excite the motor.. this will also benefit your system. if they are too big it will increase the copper losses internal to the motor.
What I would suggest is you buy a watt meter, either a digital one or analog, and select the capacitors to push maximum watts into your inverter after you get it running.
Then put the disconnect to the motor after the capacitors.. leave the capacitors in circuit at all times, connected to the inverter.
Another option is an NTC resistor, you can find one in every computer power supply. install one of them for each capacitor. then you can hot plug the capacitor.
of course if the line between your inverter and your hydro is long enough, then you can hot plug the capacitors, because the line resistance and inductance will limit the peak currents involved.
but i would not attempt to just plug in a 100 uf cap right onto your 240vac inverter. might work.. but it might trip the inverter.