I have been running Yosemite on a 2011 iMac with my regular development tools since June with the developer preview, and for a couple days with the final version, and no incompatibility seems to have arisen with XCode 6 and App Uploader.
Any Mac running Yosemite can run Windows XP-7-8.1 with an emulator such as VirtualBox or VMWare Fusion. Have not tried yet with Windows 10.
Hackintosh and Mac OSX under virtual machines on Intel PC so far do not seem able to run Mavericks, so no XCode 6. Besides, it breaks Apple EULA, which is frownable upon...