Imo, needing to adjust a saddle "as a horse grows" is BS. My horse has had the same saddle since he was 6. A good saddle can be reflocked to adjust to whatever minute changes the horse goes through.
As well, generally unless you are doing really strenuous work, having 2 saddles, a wide and a medium or MN, and a good half pad, you can fit all the horses well enough to exercise them. KN, Passier, Stubben, etc are all good saddles which will easily fit multiple horses. Many of my friends, and myself, who ride multiple horses have one saddle that fits most of them, and another saddle which will fit the more narrow guys. I'd say go for a MW tree saddle and a half pad and you can fit most of the horses, if the saddle is good quality.
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