I would think it best to make sure any ATS doesn't switch until the emergency source is within frequency and voltage tolerances. Then, if I had problems with multiple compressors trying to start at the same time, I would consider TDOs on the control voltages. By time delay switching power to the control ckts, the units would just "wake up" at different times after a power outage. All high currents would be switched as they always are. This would also help on return to the normal source, and if set long enough, help prevent short cycling of the compressor(s).