I just got some Biofine by Crosby & Baker. It says to add 1/4 tsp to 2 oz cold water and let it sit for 20 min. I don't wanna risk infection, so can I just add it to the carboy without hydrating it?
It'll be fine. You can boil the water before hand (and let it cool of course), but it is highly unlikely that something will get infected. If it says to add to water beforehand, than there's probably a good reason.