I just broke down and put an AR-15 type stock on my Mossy 500. The Hogue shortshot (12" LOP) was comfy for my wife, but a bit short for me. With the adjustable one I have on it now, it's just a matter of sliding between positions and it's perfect for anyone. As a home defense gun, I leave the safety off, but leave the chamber unloaded. My thought is, I rack it and it's live, no worries about if the safety is on or not. Therefore, the safety's position doesn't bother me at all. However if it bothers you, a Maverick 88 can be bought rather cheaply, and that's basically a Mossberg 500 with a trigger guard mounted safety, which would jive a little better with the AR type stock. Having a 2nd gun means you can leave yours the way it is instead of having to compromise.
Also, no need to go 20 gauge, unless you really want to. My wife shoots 12 just fine, provided it's not a full power loading. Federal tactical buckshot is still plenty strong, and she can control it with no problem.