My guess about the amulet is simply that the shopkeeper knew it was
kinda dangerous, but didn't realize the degree. I mean, he obviously never tried it himself. He just went "okay, that looks ominous" and made an educated guess about its nature.
As for the spells, I have a few theories:
1. Mice turning into horses is a common fairy-tale thing, so maybe there was a pre-designed spell that Twilight was using for that.
2. Casting spells on stuff like frogs and mice is easier than casting spells on ponies because ponies are naturally magical. Animals and fruit are mundane, but ponies have a magical aura that interferes with spells which change their biology. Ponies could have a sort of built-in magical spell resistance to magic that physically modifies them. I.e. age spells are difficult not due to modifying the target's biology, but because when you're casting a spell on a pony, they have their own magic that has to be accounted for to enable the spell to work, and making that compensation is the hard part. To de-age Snips and Snails, you not only have to get the biology right, but you'd have to accommodate their own magic on the fly as you're performing the spell.
edited 2nd Dec '12 11:32:53 AM by JapaneseTeeth