Eagle PCB Aggregate multiple parts into one Symbol
First of all, we should distinguish between Eagle's symbol and device. A symbol is a schematic representation, while device integrates both symbols and a package. You can have only one package for a device being used at a time, so you can not do what you want this way.

However, from what I have read, in Eagle 7, there is a feature called hierarchical design, that lets you combine many devices on a schematic into one block that you can use around your project. I think that it would allow you to combine many capacitors (or any other devices) into one block for better readability.