Is it possible to install iOS6 SDK on Xcode 6-beta?
No.
Apple only support iOS7 and iOS8, and they're strict with that. As a result, only iOS7/8 simulators are available in Xcode 6.
If you want to run on an older simulator, you need to develop on an older version of XCode.
Not possible. But: you can still test on a iOS 6 device from Xcode 6. I keep an old iPhone 4 for this (look at eBay).
EDIT: there is another way - install Xcode 5 on a virtual machine (e.g. VMWare Fusion) running Mavericks.