I'm almost embarrassed to say that we are using vastly overpowered human interfaces right now that we haven't figure out how to get to talk to each other. They are Arbor IOT-800s.
The contactors themselves sit in a large metal box with the transformer about 50ft away from where our control room is.
We have the individual systems working well and the relays don't have a problem activating the contactors. We also don't have a problem controlling the ramp rate (although it took us months to get the settings right).
Where we are stuck is getting them to work together without having to manually pause each one to allow the others to catch up. The relays we are using are built into I/o boards that came with the iot-800s and plug directly into them. I can get the models off of them tomorrow, but that is not the problem area for us.