Like the title reads: I'm curious to update to El Cap, but I want to continue working on an iOS 8-based project I've got. Can you use the Xcode 6.3/iOS 8.3 SDK and toolchain on El Capitan?
Since 10.11 Beta #2 all Xcode 6.x versions will have the "no go" sign on their icon and won't launch, stating incorrect OS version.
However ... this can be easily bypassed by locating the Xcode.app in Finder under Applications. Right click and Show Package Contents. Browse into Contents/MacOS folder. Double click the Xcode executable file to launch Xcode (it will bring up a Terminal window - don't close this window as it is the Xcode process).
So yes, it does work, but only if worked around OS X's version compatibility check.
Not a big deal really. There's far worse issues in beta #1 so do upgrade. 😉