Yes, hollow base bullets, such as a Minie ball, do need to be soft in order to function as designed to expand and fill the rifling grooves as the projectile passes down the barrel.
Also, I can see the merit of soft lead round balls when loading a revolver cylinder where you slice off a thin ring of lead as the ball is pushed into the cylinder. Although, I have used my wheel weight lead balls in my own replica 1860 Colt with no problem.
Perhaps there may be some benefit of soft lead when doing precision target shooting with precisely controlled and measured loads. Although, I am not fully convinced of that.
However, for general plinking and hunting purposes a hard lead round ball is fine in a rifle or a smoothbore.
With my .50 caliber rifles I use a .490 ball. I have never bothered to measure the thickness of my patches. I use what works, be it a commercial patch, pillow ticking, or a worn-out old shirt. One of my .50 rifles seems to have an exceptionally tight bore. I have never bothered to measure it. Instead of my usual .490 ball, I went with a .480 ball and that seems to work just fine in that particular rifle.
Neither, in general practice, do I use a short starter. Most of my rifle barrels are coned. Thumb pressure will push the patched ball down to just below the muzzle level of my rifle. The ramrod pushes it down the rest of the way. Most of the time I use a loading block, so once I center that with a ball over the rifle bore, I push the ramrod through the loading block and push the ball all the way down.
A short starter is just another piece of foofaraw that I don't need to carry.
Same with a priming horn. I prime directly from my powderhorn. I use 3F for everything.
Back in the days when I was competing in over-the-log shooting matches I used a .495 ball in my .50 heavy chunkgun. With a patch, that was tight fit, and I did use a short starter then. Even then, however, I was using my own cast wheel weight round balls.
What works for one, may not work for another. As I said above; "experiment and decide for yourself".
This is not rocket science, folks. Traditional muzzleloading is all about a technology that is hundreds of years old. Too many, I think, complicate the matter unnecessarily. Keep it simple.