My primary account includes only F-Droid apps and a few that I sideload that don't depend on Google Services. I spend 98% of my time here. I have a secondary user account with sandboxed Google Services installed and a handful of apps that require it.
So basically anything that requires google login, keep together on one account?
Some apps require Google Services and won't function without it. Most often it seems to be for notifications but it can be for other things as well. As an example I sideload the Twitter app. It works 100% except for notifications which is cool with me. Other apps just won't work without Google Services but I find that most do minus notifications.