I use rubber stamps quite often. I have a whole stash of different designs that I boiught at the craft store. I stamp my soap with them as soon as I unmold and cut and have never had a problem with soap sticking to them, but that's probably because I always press my rubber stamps beforehand into either gold dust mica or bronze dust mica and give the stamp a little tap on the side to shake off any excess before stamping. I never have needed to use a rubber mallet with my rubber stamps. I just gently press until I feel I've reached the 'sweet spot' and then I lift the stamp right off the soap. Then I spritz the soap with a fine spray of alcohol. That seems to help set the mica in place.
I have other stamps that I
do use a rubber mallet with, but those are not rubber stamps. They are made of some kind of hard substance attached to a long column of wood, and the design on them is recessed instead of being pronounced like the rubber stamps are. I cannot dip my hard stamps in mica because of how they are recessed, so with those, I like to wait until my soap a little bit more firm to prevent sticking, but not so firm that it's hard to get an impression without damaging the soap. HTH!
IrishLass