RPCs are rated in different ways. I have an ARCO, and they at least used to rate theirs with two ratings. They had a rated single motor size, that no connected motor should be larger than.
They also had a total connected load rating, which for "those" RPCs from "that" company, was 3x the single motor rating. So you could run 3 motors of the maximum single motor load, or any combination that added up the same.
Other makers, especially any that do not supply the idler, generally do not do that, and I have no idea what the breakdown is, what their largest single load is, vs their largest total. Call them and ask.
It is also true that connected motors, once running, can act as added idler capacity. This is only if they are not loaded, once they have a load on them, their capacity to assist drops rapidly.
The dust collector would be loaded most of the time*, but the machine may not be. So the dust collector would not be likely to help start the machine, but the reverse could be the case.
As for running the loads, if the 20 HP has any meaning, it should mean that the converter could run up to a total of 20 HP. But again, I do not know how they rate their stuff.
And, if they do not supply the idler, then THEY do not know what it will do either, because so much of the performance depends on the idler. After the converter is started, the idler is doing all the work, so IT controls what happens. If they do not supply it, relying on you to connect some random design of idler, then nobody really knows what the system will support.
So they will likely be very conservative, as they should be.
* The dust collector is loaded whenever it is moving air. If all the gates are shut, then it is nearly unloaded, because it is just whirling the same air, not moving it through the pipes..