Can I install used hardwood flooring?
That depends on how you plan to re-install it. If you plan on face nailing it's fine to just lay it down as though it were a ship lapped. This might sound a bit half arsed to some but I've seen reclaimed flooring face nailed down with cut nails (into the joists) and it looked very nice. You can run a bead of construction adhesive down the seams for a little added insurance if your worried about it lifting. 

If you want to install it as it was intended to be installed, you need that bottom shoulder because it's what you nail through, which is probably why it broke off in the first place. In this case you can re-groove it easily enough with a router, dado stack, or shaper just make sure to check it for shrapnel first or you may be sorry. Good Luck!