If you use WASD for movement I would not use any modifier with A for anything. If you don't use strict, then keyclone will pass unmodified A's even if not on your whitelist. If you do you use strict, you'll run into the timing problem (hold down shift, hold down A, then release shift before releasing A), causing other toons to run in a circle (edit: and if A is not bound on your other toons timing may still be why the other is not mounting if you're using strict, you have to be certain you press shift first, then press A, then release A, then release shift).

I'd just use another key entirely. I used F12 which is mapped to one my my 6 mouse keys next to my thumb, with my /fol macro right next to it.