I'm learning this software because its free! But it's also not as intuitive as Illustrator.

I learned that "brushes" are shapes and you can create and use them on the fly. (Copy and Paste)

However, for the life of me, I cannot find out how you can apply the shapes to existing path. Useful for editing paths!

Or is this a limitation of Inkscape? You have to do it right the first time!

In Inkscape you can achieve this with Live Path effects (LPEs), and also probably using some other methods and workarounds depending on the particular effect.

Try the Pattern along path, you have it as an LPE and also as an extension (slightly different results).

The Spray tool, new in version 0.48, I think, might also do what you want. Although I'm not entirely clear what you want.