Not an ideal setup but, being an experimenter, I'd try this, too. Rather than using a starter or spin-up motor the first thing I'd try is add a cap [not sure what size but I've seen info on this on the 'net] between one of the power leads & the third motor lead through a NC contact from a contactor to start the motor with no load. I'd connect this contactor coil to an on-delay timer set for more than enough time for the motor to get up to speed. I'd use a second contactor with 3 NO contacts between the 3-10 hp motor leads & the load-side disconnect switch &/or other protection equipment. The intent of the timer-contactor wiring is to allow the motor to start & accelerate unloaded, then apply the derived 3-phase power to your loads. To try it out I wouldn't even use a timer & probably just a NO push-button switch to the starting cap; I'd only want to check the impact of starting the motor on the lighting, AC & other equipment. You might find that you don't need an auxiliary motor & everything else to get it rolling.
One more thing I'd try is to add about a 10-20 uF, 350-450 WVAC "motor run" type cap across the 2 motor leads that you're applying the 240 VAC to; this'll improve power factor somewhat & reduce the motor current. This applies to the motors you'll be supplying, too. Hope this helps.