Having started ponies of that size,I would not want to put a 9 stone adult on if there was another choice.
I would rather a lighter adult or teenager under supervision,especially if it is going to be a childs pony in the future.
I have found that adults tend to ride totally differently to children and a teen is the ideal transition,they will keep a pony "straight" without being too correct.It is then easier for the pony to respond to a less capable child .
If the pony is going into the top showing world and is being "produced" it may be different.
That said most ponies would benefit from an adult on them sometimes.
No I cant really see a problem ,you asked for opinons so I gave mine
It will probably do it good to be worked, most ponies love being active.Just think of its role as a childs pony and ride it slightly less "perfectly" than you may a young horse.
I cant see anything wrong with an adult starting it, but agree if its going to be a childs pony might be worth scouting out for a competent little person to help