Hydroguard in coco or hydro only??

It's not a nutrient, it's a beneficial bacteria and of course you can! One could even possibly assume that it would work better, the medium is much more suitable to grow bacteria than a tank.

You should be fine at the same dose as in hydro.

Never used Hydroguard myself, but I often run nutes mean for hydro or coco in soil with great results. I do make it around half strength vs what the label says as the soil holds onto what ever you put into it more. So its easier to always add more later but if I make it too strong to start with, well have you ever tried flushing soil? I dont recommend it... So id try it for sure, but start with weaker doses and go from there.