Im mixing up my soil..i was wondering if i can add coco from canna to pro-mix..if so how much should i mix
You can add however much you want. Th promix is going to hold moisture longer. So depending on how fast you want the coco to dry out would depend on how much pro mix you put in there.
Yes you can no problem. Mixes like advance #4 are part coco, peat moss (which is pretty much pro mix), and perilite. I use advance 4 on the regular and love it, and find the peat coco mix to be a good combo. I read somewhere peat holds 30x times the water and coco holds 8x water, so mixing it can help relieve some watering burden. Recently though my shop was out of Adv 4 so I went with just pro mix and had some issues. I think my problem was I was treating the promix like adv 4 and watering to much, with the extra water it holds I suspect it was drowning the roots. Coco has good aeration properties so for my system I think need that. Now that Im back on adv 4, things are rocking and rolling again.