In order to support the tools I would say you only need the 10.0 .NET libraries in order to compile them.
Try following these steps:
Copy the "ArcGIS\DeveloperKit10.0\DotNet" folder to your development machine.
On the projects that you would like to be 10.0 compatible, remove all existing ESRI references and then replace them with the 10.0 ones via "Add reference" and selecting the equivalent libraries from 10.0 folder using the "Examine" tab.
Compile and you should be done. Your tools are now 10.0 compatible, unless you are using any 10.1 new interfaces.
Hope this solution works for you.