You likely will have not much trouble. Leadline saddles have very short flaps and the billets are quite high on the pony's belly. Plus, it likely has a nice round belly
I actually ended up using a 22" or 24" girth on my daughter's 11h 2" pony with a leadline saddle. I did purchase an 18" one specially for the purpose, but it turns out I do not need it. I think I have a 14-16" one somewhere around as well.