I was messing around with the asset manager so I might be able to help you out with this.
We don't know if all your assets adds up to 10mb or 800mb. To test if loading all your assets at the start uses up too much memory use this code:
mStarling.showStats = true; // change mStarling to what ever you are using i.e _starling
Run in Release not Debug. Test loading all assets in one go at the start and check the memory, click on every screen and check the memory after all of that. If the memory is fine and the preloading time at the start is not too long (test on a Mobile not desktop) then everything is fine.
If the preload time is fine and memory is fine when the App starts but is too much after loading all screens, then make sure you are disposing each screen properly.
The scaffold_mobile demo that comes with the starling download has an asset manager that loads all of the textures at the start while displaying a preloader. In the root folder you can create a second or more assetmanagers just copy the code and rename everything e.g. sAssets2 = assets2; You could have the main loader for the UI and menus then reuse the second one to load each level. Display a loader for each level and purge (clear) the level AssetManager when exited.
P.S. If you use the scaffold_mobile demo and don't need a scalefactor just put that equal to 1 (var scaleFactor:int = 1;)