Should I use Cal-Mag to condition/buffer my coco coir before I plant out?

Absolutely. Dutch Master recommends soaking coco coir with Cal-Mag for several hours at a dilution rate of 10-15mL per gallon of water regardless of whether the product states it has already been conditioned or buffered. Remember to also add Saturator with Cal-Mag at 2mL per gallon to guarantee a good soaking!