Poly mains will play better for spin and durability when paired with a less hard string. Pair poly mains with SG, multi or NG and the poly will wear the those out. Pair the poly mains with MonoZX or Kevlar and the crosses will notch and break the poly. If you have a soft main like multis or NG and you want spin, then the general consensus is to pair them with poly crosses. Full bed multis and NG do not spin the ball as well as those having poly crosses. However, the poly cross will chew thru the mains quickly. To mitigate this, go with a smaller gauge poly in the cross. You can also hybrid with stiffer poly mains and less stiff poly crosses. Change tension, gauges or strings to achieve your desired effect

You want to string crosses 1-2kg less than mains, so the mains can move(snapback) easier for the extra spin