Don't use the aon unless you really are afraid that they will fill it in partials and you won't get the entire order filled. Sometimes if I want to sell something at the end of the day I will use one so that I know I either sell the whole thing or I can try selling it the next day.
In regards to your last question, they are supposed to fill in an orderly fashion and theoretically you should be filled first. But we all know that it doesn't always happen that way. And yes, I have been filled when there are more than one mm's on the ask and my mm was below the others.
News 
Market Data 
Discover 
AI
