Not sure how much Mini Babe is actually, she may have some shetland, but I think she is a lot of the stouter more pony type of mini. I know she's registered (not AMHR) but I don't know much more about her except for she and her 6 year old are about the cutest thing to watch. In the world.
My trainer actually breeds minis for riding (for small children) and driving, and they're all of her build, some even a bit more ponyish, and she even had one full shetland for a while. They're taller and stalkier, but still do well inhand and you can ride them as well. I'm not really a huge fan of the super refined type.
Minis get ridden a lot in my area, but you never see the refined arab typey show mini getting ridden, even by very small children. I don't really think they should either, they're very delicate looking, I feel like their little twig legs would snap in two!
Anyway my view on miniatures and riding is always a case by case scenerio, and I have no issue on the more pony esque types like Babe being ridden by 6 year olds, and even briefly by lighter teenagers/adults for breaking and more advanced training.
I like your rule of thumb, but I have 2 friends that are the tall lanky type of girl (you probably know the type) and weigh like 90lbs but are like 5'7" and taller. One could reach out and touch the ears of her 14h project pony without leaning forward. He had a very large barrel so her legs didn't come off, or else jumping might've been interesting. I would prefer either of them riding them than me, even though I'm 5'1". I'm stockier, and I weigh more. So sometimes its better to just use good judgement on the mini/pony in question.
I just find regardless of weight, if you're so tall for a Mini that you're having to pick your feet up, you're immediately unbalanced and putting a lot of unneccesary strain on the animal. I've seen it in every video so far. A lot of the girls look VERY light, but the minute they pull their feet up, they're removing any chance they had of a proper training session anyway just from fighting to stay on so bad (which is just horrible for their spines).