The error I get when trying to add more iOS Simulators for testing purpose. -
"Could not download and install iOS 7.1 Simulator. Authorization is required to install the packages."
The issue also is existent with XCode 7 Beta versions.
I think it is more related to El Capitan OS version, because my other colleagues on Yosemite can install these iOS versions pretty easily on their Yosemite MacBooks.
Mine is Xcode 6.4 on El-Capitan 10.11 beta release.