Jeffrey,
I have the same situation, 25 HP home made rotary converter, 15 HP IR
compressor. Here is what I do to get things rolling.

First, the converter. No caps. I use a 1 phase starter motor
directly coupled to the 25 hp motor shaft. Starting uses a time delay
relay and a contactor. When power to the converter is energized, the
starter motor comes on getting the main motor up to speed. Then the
starter times out with a time delay relay, and the main contactor
comes on.

Second, the compressor. Make it easy to start by installing a time
delay relay to energize 2 solenoid valves. One relieves all
intermediate pressure in the interstage pipe. The other applies air
pressure to the unloaders. When the compressor starts the TDR goes
on, opening the valves, then times out in about 30 seconds, after
things are up to speed.

There may be 10 or 20 psi left in the tank. Not enough to open the
unloaders and enough to add a load. If that is the case, dump all air
pressure.

Also get some heat tape and wrap it around the cylinder bases to warm
the pistons and cylinders. That helped mine to roll easier when cold.

After my rotary converter I tapped off to a step up transformer that
feeds another panel. That gives me "home made" 480 3 phase. The
compressor starts much better on 480 that 240. I switches all of my
big motors over to the 480 panel.

Finally, start another motor. I fire up my shaper, (15 hp 480 3 ph)
before starting the compressor. The added induction helps the
compressor to roll over.

This last step is only necessary on the first start of the day when
things are cold. From then on it is fine.

Good luck. This is real experience from someone that IS there and
DOES that. Contact me if you need more info.

Bob