I have all the Android SDK Build-tools installed from revision 17 to 21.1.2. Am I allowed to just delete old packages without any problem?
I think you can delete the old build tools and that is indeed what I do each time a newer version is available. I have not got any problems by doing that. Eclipse ADT can automatically pick the latest available version in your system to build your project. If you use Android Studio though, you need to specify the version of build tools in build.grade file.