Dual boot dual partition and clean install of High Sierra
You can indeed install High Sierra without converting to APFS. I don't know how you will be running the upgrade, whether it will be by a bootable USB but you will need to make sure that you put the following in the command --converttoapfs NO.

So if you were running it via a USB stick from the command line then you could use a command along the lines of the following:

/Volumes/Install\ macOS\ High\ Sierra/Install\ macOS\ High\ Sierra.app/Contents/Resources/startosinstall --converttoapfs NO --volume /Volumes/HFS\ Volume


with HFS\ Volume: the name of your "first" partition (e.g "Untitled" or "Macintosh HD" - don't forget to escape any blank spaces in the volume name!)

I wouldn't recommended using the GUI installer itself as I believe that will convert your drive to APFS, running it from the command line allows you to enter the converttoapfs option.

Also as for dual booting you are correct as you can boot into any partition with a valid OS install by using the Option key boot modifier, or at least could the last time I tried which was a long time ago.