EDIT: Making my answer better by offering a keyboard shortcut method since my question found a solution back in the day. for even quicker safari extension disabling in or out of private mode.
ORIGINAL: A much easier way to perform this action is to enable the Develop Menu in Safari Preferences -> Advanced -> At the bottom you can check this to have a Develop menu in your menu bar for safari. Here you can select from the dropdown "Disable Extensions" which will disable them completely for both private and non-private windows but the activation and disabling of the add-ons is extremely quick and hassle free.
Making private window/tabs default to having extensions disable isn't an option in any Safari settings sadly. However this is an easy one click solution to your problem.
Again, the steps are as follows:
Safari -> Preferences -> Advanced Tab -> check "Show Develop menu in menu bar"
This works with Safari 9.0 - 11.0