Let's be clear on a couple things first, both the rotary convertor and the rotary compressor are motors, so be clear which one you are referring to in this discussion. You will from my experience find yourself running the compressor and at least one other item at the same time more than you think. That is not as bad as it sounds. The problem is both starting at the same time, which seems is going to be a concern in your situation. Worst that can happen even if only occasionally, the breaker will trip, and if in the middle of a machining operation that is not ideal.
BTW, install a $30? lightning arrestor in the single phase panel, it's really cheap protection.
BTW a 6hp convertor is woefully inadequate for this task, not sure how I missed that and maybe adequate just for the other machinery, if running two convertors is an better option.
