I like to be as kind as possible, but as hard as necessary.
It always cracks me up about all these NH gurus who spout "You should never ever EVER have to result to force or violence with a horse". If they were truly about natural horsemanship, then they would treat the horse as if it were in your little herd of 2 and you were the alpha mare. Which means, if they step out of line, they get a warning; in actual horse language, that would be pinned ears and the stink eye, human version would be the stink eye and a vocal cue of some sort. If they continue to misbehave, then they get a bigger warning; horse language = nip or paw or grazing kick, human language, firm bump with a halter or smack with an open hand. If they blatantly ignore or charge past those warnings, then all bets are off. Alpha horse will charge and remove hide and hair with teeth and hooves until they get their point across. For the human, that means doing whatever is necessary to do the same. If that means having to take a whip or a crop or a lead rope to them, then so be it.
My mustang, who is the alpha of my entire herd, including 2 drafts and 2 crosses, is great for putting young horses with. Not only does he protect them from the bigger horses, he teaches them how to be a proper and respectful member of his little equine society. My older draft cross (pictured in my avatar), when he was about 6 months old, started getting nearly unmanageable because he thought he was the alpha in his little herd (mustang was not housed with him at the time). He bossed his momma around and bossed all the others he was housed with. When he's already almost as big as a 2 year old standard sized horse, that's tough to deal with. I was struggling to keep any semblance of control over him and it was a constant battle to keep him grudgingly respectful of my space and my orders.
I finally had the idea of putting Dobe (mustang) in with him. When I introduced them at liberty in turnout, the first thing Rafe (draft cross foal) did was run up, arch his neck, squeal and paw at Dobe. When he didn't immediately back off from a pinned ear and the stink eye, Dobe proceeded to grab him by the scruff of the neck and basically tackle him to the ground. He held him there for a few seconds, gripping Rafe's neck with his teeth and leaning on his ribs and neck with knees. When he let Rafe up off the ground, the attitude difference was amazing. Rafe immediately turned into an upstanding, mellow, and respectful foal and I never had another lick of trouble or disrespect out of him.