Hackney ponies have snappy gaits or should, almost like mini saddlebreds. I weight roughly the same as you, (not sure about height but I'm 5'9") and I wouldn't ride her personally. I trained a 10 hh welsh A gelding, that was high strung and he loved work, once he could ground drive, I have a smaller friend (90 pounds at the most) lay over his back and eventually rode him while leading, than lunge line than on her own. I very got little kids riding him before he sold (to a driving home), but he was very face and would have needed a experienced or brave little rider for sometime before I would even think of a beginner on him.
As a saddle I used a pony pad not a real saddle because his back was too short for anything with a cantle and a teen to ride him at the same time. Sometimes a pony that responds instantly isn't a good thing, if your little brother pulls back at a fast trot trying to keep his balance and she stops just like that it might not be good. A beginner horse or pony will almost hesitate into things as not to jar the rider off balance.
Might want to mention my little gelding thought he was stud muffin (he was gelded as a foal), so he was very showy and loved to fling his legs out as far as they could go.