I am working in Illustrator CS5. I want to select 100 objects (circles) and replace each of them with a 100 other various objects, each circle being replaced by one of the various objects, keeping the size of the originally selected circle. So in steps:
Select 100 objects (ex. circles)
Choose which objects to replace them with
New objects takes place and size of selected circles
Each selected circle becomes one of the newly selected objects.
If you have all the 'various objects', you should be able to use Actions panel to make them all the same size. If you already know the size for the 'circles' you can start with creating one 'master' symbol, for size measurement. Create 100 artboards and place the 'master circle' as a locked symbol on each artboard. Now populate your various objects on all artboards within the bounds of the master circle. You can resize 'various objects' manually as you place or resize all at once if they already are the same size. Designate each 'various object' as a symbol. If your 100 circles are also designated as a symbol, you can go to Symbols panel options and choose 'Replace Symbol', select one circle-replace with object, continue. IDK about any way to do this automatically. If you want to replace a symbol and select one at random I suppose this is possible by scripting.
I have quite a few years of Illustrator experience under my belt and I don't know anyway to do exactly what you are asking for.
However, There is a spray tab in illustrator that lets you duplicate quickly any image you have created. Using your example, I would create 100 circles and then save them to my Symbols tab. I would then select my spray tool and multiply my 100 circles. This would work almost just as effectively as what you are after.