The hardened pushrods are the performance pieces that you need when you have guideplates. Most small block Ford pushrods are not hardened as they don't need to be.
Yes, you can replace a single pushrod. There are different sizes. Some are for different deck height and head combos. Others for adjusting the lifter preload. Most of the time just a stock pushrod for the year engine you have will work just fine. And there is a lot of overlap between sizes as well. So it isn't just one size that works, usually there are several slightly different versions that will work.
The pushrod is often the fuse in the valvetrain. It failed, usually because something else is having a problem. But sometimes it was the problem.