What western style saddle does everyone like to start greenies/young horses in? Their backs can change so much and some may only be with me for a short time so I am looking for a saddle that generally will be able to fit most horses well.
I am considering treeless vs adjustable vs others?? I really like the idea of the DP “flex fit” that can be widened or narrowed with a tool - does it work as well as it seems like it would to fit a variety or horses? I have never ridden treeless, but this seems like a good potential option - can you really achieve proper spine clearance with correct padding?
Wintec western saddle with full quarter horse bars. Light weight, fits almost anything with a change in saddle pad, practically indestructible, and cheap enough that it won’t make you cry when a horse rubs it all along the side of a round pen panel.
I would not recommend a treeless. They can sore your horse and if not fitted properly and have excessive pressure points. Maybe instead get a flex tree, they are known to fit a wide variety of horses. I have a Circly Y Kelly Kaminski WIDE flex tree that fits my gelding wonderful. If you do get a flex tree I would recommend a STANDARD size unless told otherwise by a professional saddle fitter. Flex2 trees are also super lightweight. That would be my recommendation for babies. For a lightweight flex2 tree, Circle Y is the best quality in my experience. You don’t have to manually adjust them, they adjust to your horse as you ride. Good luck! 
I would not recommend a treeless or adjustable tree saddle for starting babies (really ever, but especially with youngsters). Nor would I go synthetic. You want a high quality, balanced leather saddle on a sturdy, well-built tree. If you are mostly starting narrower type horses, semi-QH bars might work, but full QH bars will allow you to pad up temporarily on the ones that need it, whereas the narrower saddle will never fit the wide ones.
As far as what style/type of saddle, that really depends on your own preferences. Personally, though, I like to start babies in something that has a back cinch for (a) additional stability and (b) having that be just “part of being saddled” and not a big deal that gets sprung on an adult horse years down the line.
