These are totally 2 different products and there is no relation between them and the data saved with Save Game Pro won't be loaded with the new system and vice versa, also there is an upgrade available from Save Game Pro to the new system that allows existing users to switch to the new system comfortably for their new projects.
Yes, but not directly, you first need to migrate your data to the new system, check out Migration guide for more information.
No, Save Game Pro will be in maintenance mode which means it will only receive bug fixes and patches, no new features or breaking stuff, it will remain stable as is, but we recommend you to use the new system for your new projects instead of Save Game Pro.
Save System's minimum version requirement is Unity 2018.1, and has currently been tested until 2019.3.
We try to support new versions of Unity as soon as possible, but using Save System with Alpha and Beta versions of Unity is not guaranteed to be compatible.