How easy is it?

How easy is it to take an sps frag off a plug and glue it to a rock? I guess i should first ask, are you allowed/supposed to do this.

 

depends on how strong the frag main stock is. usally you can carefully snap it off or use a flat razor to pry it off.

 

Depends how it's attached to the plug and how well encrusted it is. If you have a picture of it or a good description of what type of plug it is and how it's attached and whether it's encrusted someone can probably give some specific recommendations. Also, how long have you had it and has it grown any (trying to determine if it's healthy/acclimated enough for safe fragging if necessary).
You definitely can do this in some circumstances, the details will determine whether it's a good idea right now and what the best method might be.