Gingerbread,
Sorry, I wasn't trying to say you don't know what you're doing. I respect anyone who studies and listens to others. However, if I were to look back 20 years ago... It's amazing how much I didn't know back then. Certain things that I would have argued about because of being so certain, have turned into all together different thoughts about the same thing. That still doesn't mean I am right or you are wrong... It just means that is is very likely that we all have much more to learn and we will never learn it all.
Pitch counts, rest between pitching, and other subjects are extremely important, but have nothing to do with the original topic.
When I hear someone say that curveballs and sliders are determined by what the ball does, I understand what is meant, but these are two pitches thrown different ways. While some might throw the curveball differently than someone else, they are both throwing a curveball. Those few pitchers who throw both pitches... throw each pitch differently!
Also, in order to increase arm strength the pitcher needs to throw hard. Without doing that he will not see a lot if increase in arm strength or arm speed. The amount of effort required to throw hard differs from one pitcher to another. That is because they are not all equally talented or one has much better mechanics than the other.
There is no athletic event that I can think of where people improve without effort. You do not improve accidentally. It is my belief that every top level draft pick pitcher has in fact tried to throw as hard as he can, whether there was a radar gun present or not. The other issues of over working the arm or not getting enough time between pitching performances or bad mechanics... Are the things that cause most all of the arm injuries among physically mature pitchers. It's not that they throw harder, they need to throw smarter.
The curveball thrown correctly (which is rare) is a safer pitch for a young arm than sliders and split finger pitches. Throwing the fastball properly is safer than all breaking balls. The true slider or cutter could be called throwing the fastball incorrectly. That is dangerous for under developed young arms! Personally I think the split is the most dangerous for a young pitcher. Especially if they don't have long fingers.
There is a grip slider that some use that allows a young pitcher to get a decent break at decent velocity without much strain on the arm. The traditional slider that requires curling and pulling the middle finger down is more dangerous and places much more stress on a young arm. (IMO)
BTW, while I can see some value in mastering the curveball earlier on... The slider is a pitch that can be learned and used later on. It is not a hard pitch to learn and often becomes the second pitch (even the best pitch) for older kids who throw with lots of velocity. The problem with the low arm angle is that the curveball is nearly impossible to throw. The "grip" slider can give the low angle guys a good breaking ball. Best about the low arm angle is it should provide a lot of action on the fastball.
All that said... There are no safe pitches to throw. They are all dangerous. Some more so than others. Recuperation, rest, mechanics, conditioning, strenth, genetics, brains, luck, etc. it is all important.