to be honest, as far as I know, we don't even have rules for breeds d:
As long as it is a horse or pony breed that is at least 14.2 HH (around 147 cm) in reality it is possible : D So Gypsy Vanners are definetly allowed c: (we actually have quite a few of them, at least as NPCs xD)
only thing that currently is not allowed are small pony breeds below 14.2 HH (shetlandpony, falabella, american shetland, etc), since they are just too small D: