It's so hard to have two good things! LOL I can't tell you what to do, of course. I can just share my experiences. I'm a transplanted AO'er. I did "bring" with me a few favorite books, but the approach here at SCM just seems to dovetail with our needs and personalities better.
If you think there is not enough reading for the 10yo, then simply add more. This is SO EASY! Either come up with a list that you just hand to him or her to have read during the year, or assign a few extras. The Bookfinder, the AO lists, the Sonlight lists, etc. can all yield more possible titles for your child.
It doesn't really matter what module you start with. You could make a good argument for starting at 1 and working from the beginning, or also for beginning with 4.
I have to tell you. I used to keep all my kids together for history, and then went to AO and separated them, and am now trying to draw threads together again. I missed the all-together terribly! History study had been a vital and exciting part of our lives, and it suddenly became humdrum--go read your separate books and then narrate to mom. Whoopee. Sigh. It used to be so much more fun! Not to mention easier for me to keep track of! I must say that trying the "all separate" route is I think one of the biggest regrets of my homeschooling career.
I have to say, it is much easier for me to start with simple and then add in a little bit more when I need it, than it is to begin with the more complex and then try to figure out "how much" I need and try to simplify it down. It is so easy to just say "OK, your reading is only taking you fifteen minutes a week, let's find you another book!" than it is to say "Well, the schedule says you ought to be reading ALL of these books this week, and I don't know which one to cut out, so you need to finish them. I know it's bedtime-finish them!" LOL I guess I used to have a "it's on the schedule so YOU HAVE TO DO IT!" mentality!. That's why the Organizer has been such a blessing to me. :-)
So--take the best of both worlds if you want. Come to sweet simplicity for your basaic framework, and then if you want to "migrate" a book or whatever with you, you can. We liked doing Plutarch so much that we've kept that; we do quite a bit of the nature readings on AO because we just love them.
This is what has worked for me anyway. Take with a grain of prayer! :-)