Tac star is an acceptable brand for the side saddle but I wouldn't get the pistol grip by tacstar. I really like the Pachmyr vindicator grips and forend they are comfortable but a little more expensive. If you want a pistol grip with a removable butt stock i believe ATI makes a decent one.
My brother has one and we took his stock right off and just left the pistol grip. It has one screw in it i believe. The stock being removed or not would not put the shotgun in a different weapon class unless the abrrel was shorter than 18 inches in which it would already be in that class anyway.