Well, I would put the plastic bioballs in the bottom and the sponge on top. What I will use is either the ceramic rings or the bio-chem stars and they will be on top without and bag, assuming they will stay there. If they won't, I will put the on the bottom.
It actually makes sense, to me, to put them as the first line of filtration. They will take out the larger items that need to be broke down and being they have more bacteria; they should break it down better and not impede water flow.
I haven't used any yet but I think they ceramic rings and the stars will be much better simply because the bacteria has somewhere to live and not get washed off in a simple rinsing in tank water.
The bioballs, I don't know for sure how big they are but in a AC 20, I don't how much room there is going to be and a second sponge might actually provide more biological surface area than anything else.
If the bioballs are small enough to put 6 or more in there, then I would use them and only rinse half of them if it becomes necessary. Lots and lots of options, again, this is why most love the AC. Once I get my ceramic rings and my bio-chem stars, I will be using them as I do like the filtration the double sponge gives but I don't like the water flow loss in less than 1 week. At times I have to rinse the sponges twice a week instead of only at water changes, once a week. This is just another one of those glory necessities of a 10 gallon tank.
One other thing, I don't know why you are using carbon but I have been told time and time again it is not needed and is really a hindrance if you have a planted tank.
Ask away, that is what I did when I started and I still do it. Never a worry about trespassing on a post. It is amazing how often a post actually goes way off topic. You had a good question and I wish I would give you better answers with how many will fit of all types, the surface are and the biological capacity but I am not that far into it yet as I have only had mine for 3 months or so.