First, that's a solid choice for home defense. Futher, I applaud you for getting some training. Keep that up.

As for your questions... Blast has it right. Here is my take:

#1 Low recoil 00 buck for your 12 guage is an EXCELLENT choice.
*it tends to pattern better
*the kick reduction really helps second shot accuracy (et al) for most shooters
*it slightly lessens over-penetration in the home
*it takes off just enough to make a nice difference without sacrificing real-world stopping power in nearly all situations

#2 The Knoxx stocks are interesting and some combat shooters swear by them. But for most folks, a top quality recoil pad, or better yet, a stock wit a built-in pad is a better and cheaper choice. You migh consider something with the Limbsaver pad and/or a Speedfeed stock. Heck there is even one that will hold 4 extra rounds so you can talk yourself out of bolting on the silly side saddle!
One more thought, while the specops stocks from Knoxx do reduce the felt recoil, they do so with movement. Depending on what you are doing with the gun that can be an issue.

In any case, the Mossberg is a solid choice. Good luck with it.