Jim,
Adding weight out in front of the gun (like a magazine extension) might not be a good approach, no matter what gauge it is. My DW (5'4") dislikes magazine extensions intensely for that reason- she says it makes the gun too hard to support. She also much prefers a field-length forearm to the shorter law enforcement style, and her preference is for a 12.5" LOP stock with a good recoil pad. House guns here are built to suit her, with furniture as above, 18- 20" barrels, 4-shot Sidesaddles and detachable lights (Streamlight M3 or LEDWave Z-5) on rails.
She tried the 870 Youth Express gun in 20 ga. back when she was shooting 3-gun, and decided to stick with a 12 ga. and low recoil loads.
hth,
lpl/nc