I mix ammo in my mags. 3 hydra shocks then 3 FMJ. 15 rounds for my 92FS.
Mixing ammo is okay, but the backup mag(s) should be ball ammo, since they are with you incase of a serious firefight. The first few rounds out of my Bersa Thunder 9 Ultra Compact are hollow points for close-in max damage, quickly. All the rounds in the 16 round backup mags are ball ammo, one of 147 grain and one of 115 grain.