I seem to remember that Paso Finos have a back that fits more like an Arabs's back. So, even though the horse is small, the withers are so-called "mutton withers". You might want to do some internet research on saddle fitting for Paso Finos before you assume it is a medium. Most Arabs need a wide or med-wide saddel to accomodate their low, wide withers. I am NO expert, just something I remember reading somewhere and I could be off on this.
Read, research before you buy . It sucks to buy off the internet and find it doesn't fit. Been there, done that.