According to the seller that size works on everything from Newborn dolls to Barbies. They don't have any other size. Yikes, I'm sorry I'd be scared too on those guys. That loos about to be a size 4 or 5 needle, something I'd use on only larger dolls like Blythes and maybe A styling Pony if it was really problematic.
However----I agree with CSA---try it on a less rare pony first and enlarge the holes with a pushpin. What I used to do was go back and forth with it until I got to the middle. So put push pin in root at base of neck, leave in for 20 seconds, take out and move the push pin to root near top of head, Root your hole with hair at base of neck, then take out push pin, move it back to next hole on base of neck, Root Hole with hair near top of head and rinse and repeat until you get to the middle. The holes will shrink back to normal size after you are done. When you root, try to go sideways with the needle prongs (so you avoid splitting the scalp) but I think if you enlarge the holes enough you should be fine. I've also used large leather needles if I need a bigger plug hole in the past, worth a try!!!