I'm in the same situation as you. I have a partial lease on two horses - one an old-tyme flat-backed Morgan built like a tank, the other a TWH with hollow shoulders and high withers. I also ride my sister's petite old TWH, and go on trail rides with a friend's average-shaped mutt. I ride English, and all these horses are normally ridden Western, so I have to use my own saddle. Same dilemma. I have one saddle that I use for all of them. Obviously, the fit isn't going to be ideal for all of them, but since I don't ride any of them very often or very long/hard, I've been able to make it work without any obvious discomfort, injury, or obvious horrible fits for any of them.
The key for me was to be sure to get a saddle wide enough to fit the widest of them. If a saddle is too wide, you can pad it up to some extent, but if it's too narrow, nothing can make it fit better. Then I had the problem of what to do about the high-withered horse. I ended up with a Passier that fits the widest horse very well. That width should make it sit too low on the high-withered horse. However, the saddle is cutback, so the withers are clear. Then I fill in his hollow shoulders with a Cashel correction half-pad. Cashel Company
With the half-pad, the fit is pretty darned good. I've seen no soreness or hollowing from him.
The other two horses are OK with it as well using various combinations of thin pad, wool pad, and half-pad.
not ideal, but it's working,and the saddle fits me like a dream.
I know someone else who also has 1 saddle for several horses, and is using a Wintec 2000 with the easy-change gullet system. Also not ideal, but so far, so good.
I know there are several other brands that have adjustable gullets as well. I think HDR and Thorowgood both have adjustable trees or gullets. That might be an option for you. If not, just be sure that the saddle is wide enough for your widest mount.