As far as remoddeling spacemarines, the answer should be NOOOOO. It goes against the Codex
The reason space marines dont need a stock is because they are so bulky, especially in power armor, that they can hold a basic weapon in one hand with no penalty and still fire full auto. They don't even need to brace for using a heavy bolter. They are that big and strong.
As for all other things, I would like to see many cool pictures of marines using cover, as THEY DO. One thing I remember hearing about the space marine game, although kinda cool, was the phrasing along the lines that "space marines never use cover". Ok, granted I could see that applying to Space wolves a lot of the time (though even they use cover) or the World Eaters before they fell. But most marines use cover as much as possible.
Sure, if facing a bunch of tiny imperial guardsmen with lasguns, that probably wont penetrate their armor, yeah, they wont stay behind cover for that. But heavy weapons, tanks, bigger groups and so on YES they will. Space marines arent stupid, not even Space wolves.
That guys unusual way to wield weapons seems like it would probably reduce recoil a bit more, but honestly I don't think it seems like a good idea anyway. It would, it seems to me at least, be more unbalanced to run around like that. In CQC you cant punch or do anything with that arm either as its already at its full length and if youre in a trench or other such thing, you can probably just lean the weapon on something else to get more stability.