I downloaded xCode 6 from https://developer.apple.com/download/more/, but it crashes after start.
ASSERTION FAILURE in
Details: Error looking up handler class for extension
'Xcode.IDEKit.CmdHandler.ShowViewFrames' in plug-in
The issue here is that the simulator runtime and modern Apple operating systems are incompatible. The iOS 7.1 simulator runtime is only supported on Yosemite and below. So even if you manager to install and run Xcode 6, you will still not be able to run the simulator runtime.
I suggest splitting your hdd into a second partition and installing Yosemite or Mavericks on it, and using Xcode 6 there.