Some food for thought. if you already have 1 rotary converter with or without the starting assistance of a pony motor you can parallel in idler motors till kingdom come as long as your line amperage is sufficient to handle the per phase load amperage of the equipment which is going to be run off of the converters.
I often use the motor on my end mill as a booster idler for when I need tr run heavy loads on my lathe and vice versa. I do this because my converter is the same size as the motors on the lathe and the mill which means either must be started under no load. Ideally I would have a 5HP converter.
Here is a chart which shows the maximum amount of installed HP which can be run on a given sized converter as long as all motors are lightly loaded and not started at the same time. the chart also describes how multiple converters may be linked together for starting larger motors
Attachment 21883
If I had some large capacitors I would be tempted to see if I can get my little 3 hp converter to start an 8 hp inverter rated motor that I have and allow it to run as an idler in parallel to see if I would have sufficient power to rower up a 240 to 480 v transformer that is needed to run my 7.5 HP Cleereman index drill now that the electric co has installed new 2 ought feeder lines to my meter as opposed to the old #4 AWG lines I had before. I might have to add a 3 hp idler before the 8 hp but that would or should according to the chart give me the equivalent of a 14 HP converter