With my 2 Walkers I've found the 6 yo needs a gaited saddle, with plenty of room for withers and shoulders (found a custom made on line, it's half price of catalog saddles). The 3 yo is still slim and my flex tree trail saddle does fine. I agree with the others on bit, too--if you're showing you need particular bits, but in training I move from rubber snaffle to full cheek snaffle to Tom Thumb to . . . whatever works best for each horse. A couple of good sites/catalogs to study are National Bridle Shop (.com) and The Winner's Circle (.net)--not trying to advertise for them, but I found them very helpful in studying on styles, gaited/TWH possibilities, and some inexpensive bits!
Good luck. Oh, and some places now let you use a loaner saddle to see if it works on your horse!!
Happy trails, Renee
"Bread may feed my body, but my horses feed my soul." ~~anonymous