Foot stomps are a product of wall fighting in the cage. You rarely have a situation where you would even have a chance to utilize them.
I briefly looked at Glory's rules and didn't see foot stomping as illegal.
foot stomps are not listed in the fouls of either Glory or K-1. So technically it is legal.
However I was surprised to read this in the Glory rules just now: "All techniques targeting the back of the head or back or the torso"
Not allowed to attack the torso? why the hell not?