Can I install both High Sierra and Mavericks on my Mac?
You do not have to use APFS to run High Sierra. Although, I believe if your internal drive is  a SSD, then the High Sierra installer will try to convert the existing El Capitan partition and El Capitan Recovery partition to a APFS container partition.

Upgrading El Capitan to High Sierra should not effect Mavericks.


  Note: Mojava (macOS 10.14) will be released on September 24, 2018