If it's a small pony harness and your B Mini is a big B, up closer to the 38" mark rather than 35" the pony harness may fit. The people I know that have tried to make a pony harness fit a Mini had to punch a lot of holes & even buy a couple replacement straps that were smaller/shorter.
Depending on the kind of head your B mini has, a pony bridle may be way too big. We have a couple B Minis (36-37") that have bigger heads and need a pony bridle; then our Ice Man, for instance, who is 37" has a very refined head, and he fits in a Mini bridle just fine.
Keep in mind that Mini harnesses do come in different sizes. The first one we bought just said "Mini" in the catalogue, but turns out it was meant for 32" and under. We resold it & bought one from Ozark Mtn. It fits up to 38" and will adjust down for the taller A's as well. Then some are for 30-34", and you can buy some that are specially made for the tiny horses.