Starting on shortboard?

I'm sure questions like this have been asked billions of times, but here it is: I've surfed a few times (and loved it!). I do some sort of workout almost every day, and am in pretty good shape. I have very good balance, but I have noticed that quite often, when I pop up, my feet aren't even on the center of the board, which makes me fall. (I have been working on that, however, and it seems to be improving) I know it's much more challenging to ride a shortboard (especially someone relatively new to surfing), but for someone with good arm strength and good balance, would it be possible for me to buy a used shortboard? The longboard that I learned on was far too bulky for me (I'm a girl, and about 5'3" and around 100 lbs.) I'd guess that the longboard was probably around 8 feet long
but I really have no idea. With enough practice, would I be able to just go right to a shorter board?
Thanks!

Thanks!
