lp0101 That is correct. If an app uses Play Services for notifications, all you have to do is install Sandboxed Google Play, grant Play Services a battery exception and then install the apps you need.
Installing an app that depends on Play Services and then installing Play Services won't work, so the order matters here.
Other apps implement different ways of receiving notifications. For example, if Signal notices that you don't have Play Services on first install, it'll use its own websockets implementation, which uses a background connection to give you notifications. While that works fine, it's more battery intensive, whereas Play Services is a method of delivering that is shared by many apps, so it's much more efficient.