My saddle doesn't fit my new boy perfectly. He has a flat back and little shoulder muscle. Since he's just being put back into work, I'm using a half pad with front and back shims to help the saddle fit better. Plan to wait a few more months to see how he develops before buying a saddle to fit him. I wouldn't recommend a half pad for a saddle that is already too narrow. It's ok to use for minor balance adjustments, but you're risking back problems or pain when using a saddle that's too small.
I've spent most of my life riding horses. The rest I've just wasted.