APFS & OS Extended Journaled drives in same Mini?

I've purchased a late 2012 Mac Mini from eBay in order to keep working with files/programs in Sierra from a 2011 MBP which died. I'll be transferring the SSD from the MBP to the mini and am confident everything will work fine afterward as I did much the same thing from a dead iMac to a 2012 mini. The PO of the mini is shipping it with an SSD running a fresh install of a more current OS (Catalina I think). Sierra meets all my needs nicely and I've avoided upgrading past it. That said, since I'll have a mini that can hold two drives, I'm thinking of leaving the Catalina drive in and adding my Sierra drive to have a dual-boot machine so I can dabble in Catalina.

I'm not sure what will happen if I put an APFS drive (Catalina) and an OS Extended Journaled drive (Sierra) in the same mini. When I boot into Catalina, would it convert the internal Sierra drive (and any thumb drives mounted) to APFS automatically? I wouldn't need to move files between the internal drives or OS's but even so, I don't know enough about Catalina/APFS to know if they would play nice with each other.

Also, my Time Machine external drive is OS Extended Journaled. Will it be able to back up the mini if one of the internal drives is APFS?

Appreciate any insights.