This thread might be helpful for you.
My younger daughter just turned 13. We recently tried a new setup with natural gut 16g main and copoly 17g cross. She likes it a lot. Good blend of control and power.
Before this she used Poly16 main and NXT 16 cross. Multifilament cross string would break after ~2-3 wk. She played 2 hr after school 4 days a week and plays tourn. 1-2/month+ some practice matches. Good thing about Multi is it will break so you will not have to worry you leave poly too long. We never tried Multi main. From what I read in several string threads about poly-muti hybrid, many people said muti in main broke too fast. Most put multi in cross.