I'm developing an universal application using Xcode 6.0.1 universal storyboards. Xcode generated for me one storyboard that will be used for both iPhone and iPad. To get this worked I must use size classes.
So my questions are :
If I use size classes, my application will be compatible for iOS6 and iOS7?
If not, what is the solution to make it compatible with iOS6 and iOS7, using Xcode6?
But it didn't explain the compatibility of the universal application in iOS6/7
It will be compatible with iOS7 for sure. Have not tried it with iOS6. It's easy to try - if you already have a project - just run in iOS6 simulator and see if it works (I suspect it will)
The only one caveat - you still need to use launch images for pre iOS8 versions. Launch xib file will work only for iOS8. So in your case I would recommend to use both image and xib file.