How long and thick is your snake? He could possibly take a small rat if he is big enough. Secondly,burms are meant to grow and fast! Especially in the first year so I wouldn't take it as a surprise at his size on his feeding schedule.
How big of an enclosure do you have the snake in right now? ONce we know this we can help you give an idea of when he should be moved into a larger enclosure.
Yes. You can tell the sex at this age. But you may want to get some experienced to probe it and if small enough pop it.
I would recommend not feeding live. Prey kill works fine but be prepared to prey kill large rats and even rabbits. You can always try and convert to frozen/thawed prey.
If you have the room,money and time for the larger female go for it. Just remember at her size a good sized enclosure would be 8x4x2-4...this size would last her quite a few more years so why not jump up to it quick as possible. Also you must know the fact that feeding the snake will cost a fair amount.Unless you have a hook up somewhere.
www.rodentpro.com works well from what i hear.
At 9' feet she is quite large already and I don't think she will be growing too quickly. If you kept her own appropriate prey item or items at about 10-14 days she will grow and a nice pace for you.
I would recommend finding a friend who is not afraid of snakes who is able to help you when your lil guy isn't so little anymore and if you decide to buy the 9 footer make sure you have this friend. Always better to have an adult around just in case. Just make sure your kids know who to call for help if you take it on alone and something goes wrong.
Hope this helps