Nothing forbids the use of a start capacitor in series with the PTC. If the motor design allows it, it will increase start torque due to the additional phase shift of the current passing through the start winding. I have used a PTC with a 80uF start capacitor in series to increase starting torque on a Danfoss NL11F R134a LBP compressor. This was necessary to use the compressor with a TEV instead of a captube.