How to set "Input Sources" for all users at the same time
The default input source is set in ~/Library/Preferences/ByHost/com.apple.HIToolbox.*.plist (where * is system_profiler | grep 'Hardware UUID').

You could either copy the file to other acccounts: sudo cp ~/Library/Preferences/ByHost/com.apple.HIToolbox.*.plist ~/Users/username/Library/Preferences/ByHost/.

Or add it to the User Template directory so that it will be included by default on new accounts: d=/System/Library/User\ Template/English.lproj/Library/Preferences/ByHost; sudo mkdir -p "$d"; sudo cp ~/Library/Preferences/ByHost/com.apple.HIToolbox.*.plist "$d".

Note that the UUID will have to be changed if the file is copied to another computer.

You can view or edit the property list in a text editor by converting it from binary to XML: f=~/Library/Preferences/ByHost/com.apple.HIToolbox.*.plist; plutil -convert xml1 $f; open $f -a TextEdit.