We worried about the same thing with Savannah, but amazingly, she's only fallen out of the bed once in the last 9 months or so.
We did start out with one of those mesh side-guards, but we finally took it off after a few months because she was sleeping up against it and we were worried it might break. And she seemed to calm down a lot more at that point.
Only other suggestion is maybe start out with the mattress on the floor for a bit so she can get used to it. We did that for a couple weeks. And actually, she's now back on the floor for the summer because we moved temporarily and didn't take any of our furniture, and now she's gone back to the crazy sleeping--she has a crib mattress on the floor and sometimes is on it, sometimes off, sometimes using it as a pillow.
But she isn't waking us up most nights so I don't really care where she sleeps