I have not worked with green minis who have not yet worn a bridle, but have worked with horses and minis who are hard to bridle or headshy during bridling for whatever reason and I try to sneak in a tiny treat along with the bit, just so their association with it becomes positive experience. Once they get the idea it's ok and the bridle is not bad, I stop doing it or only treat once in a blue moon so they don't get dependent on it.
Don't forget to make sure the teeth are good with no problems that could interfere with the bit. Wayward teeth can raise heck.
I have a mini who is being restarted, he was worked all summer ground driving and hitched a few times and he still has a "busy" mouth when he gets nervous. So it can take awhile. He really hated a jointed snaffle but is happier in a mullen. I may try something else in the Spring but Im giving him time. I wish they could talk and tell us their preference, it would sure take the guesswork out of it!