I'm like rabbitsfizz, I don't consider it safe for any horse--I used it once shortly after it came out, back in the days when we had only Morgans. Our horses were on a good deworming program & I was reasonably sure they didn't have a huge load of encysted strongyles. I wasn't concerned with the medication causing neurological symptoms (though in truth at the time I don't think there had been many reports of that problem yet). In actual fact the horses wouldn't be overdosed on one tube each.
If I know a horse did have a big worm load--or if I even suspected he might--and certainly if I didn't know the horse's deworming history, there's no way I'd give him Quest....nor would I give him anything else that would cause a large die off of the encysted strongyles. That's only common sense. Large die off = intestinal damage = colic. That part of the Quest danger isn't restricted to Minis--I've heard of full size horses dying of colic after being dewormed with Quest.
I used Quest that one time, and overall wasn't all that impressed with it. For administration I prefer the paste or liquid varieties over the Quest gel, and as far as effectiveness I saw nothing that indicated it did a better job than the Eqvalan I used the next time. Shrugs. Maybe it did work better, but I have no way of knowing that. I won't be buying it again.