As a big horse person---I've been in big horses a lot more years than I have been in Minis--I have to say that in my view Mini people are more cautious and worrysome for a very good reason when it comes to putting big horses in with small horses!
You, katie, are subscribing to the theory that because it hasn't happened to you--or been a problem for you--then it isn't a problem for and won't happen to anyone else.
I've had Morgans that could be with the Minis, and I've had Morgans that couldn't be with the Minis. It would be foolish for me to say to anyone that because my Morgan mare Ashley gets along well with the Minis then anyone on this board will be safe in putting their Minis in with their big horses. If anyone has a big horse like my Morgan mare Echo it would be a fatal mistake to put a Mini in with that horse--Echo would like to kill any Mini that she can reach. She just doesn't like them and won't tolerate them.
I have been to farms that had Minis in with big horses...big horses of all sizes/ages....and I have seen how they get along together. I have to say that in some cases the owners have been very, very lucky to date. Why? Because their big horses haven't yet seriously injured any of the Minis. Just because it hasn't happened yet doesn't mean it won't--tomorrow or next week or next year one of those big horses may very well connect when they kick, and will break a leg or a jaw on one of the Minis. I guess some feel the odds are acceptable--one death in 5 years maybe isn't important to some. To others that one preventable death would be devastating, making the odds not worth it to them. People need to be aware that it can happen, even if it hasn't yet happened to some here.
And yes, Minis can hurt other Minis too--but a kick from another Mini is less likely to break bone. And even when horses are all the same size, if there's one that is being especially mean to another of his own size, I still separate them--horse play is one thing, but I'm not going to have one horse terrorizing another, even when they are the same size.