Running 2 OS on a SSD disk in a MBP some how to questions.

Presently I have installed two versions of Mac OS and one for other information (non-OS).  My SSD has 500GB and is thus partitioned in 3 sections.  

·     One OS is GUID Partition Map and is APFS formatted.  

·     The other OS is Mac OS Extended formatted.

·     The other non-OS partition is also a GUID Partition Map.

Let me first make some definitions so we are all on the same page for terminology.

• a disk is the top level, the whole storage device, in my case the 500GB internal SSD,
• a container/partition is a fixed-size division of the disk,
• a volume shares the space allocated within a single container/partition with any other volumes within it, thus normally has variable size up to the maximum of its enclosing container/partition.

Ok so presently I have 3 containers/partitions.  

My questions:

1.    Can I use 2 of these containers/partitions and having 10.15 on one and 10.13 on the other?  Would this work?

2.    Do I first need to reformate the whole disk into APFS? Or would the installation of the OS do this for me?

3.    If the instal does this would it do the disk or the containers/partitions?

4.    Is it a problem if 2 containers/partitions are APFS (the OS ones) and one is not (the non-OS one)?

5.    Anybody have some other brilliant tips?

We say thank you for your help.