Dual boot dual partition and clean install of High Sierra iMac, model has been verified
Dual boot machine.
HD is SATA
Two partitions. Second Partition has Yosemite. First partion old.


I want to wipe first partition and install High Sierra.

Plan:


Download High Sierra and create bootable install as per osxdaily instructions
Using option key, boot into High Sierra USB stick.
While in High Sierra, clean and format first partition, as HFS+ partition.
Install High Sierra on first partition.
From articles online, High Sierra will not convert SATA partition to APFS


Questions


Format first partition as HFS? I believe from several articles I will have option to install High Sierra w/o AFPS, correct?
Dual boot will survive a reformatting of first partition? I come from Grub background with MBR, that's why asking question. My understanding is option key will allow booting to any partition with valid osx.


Point 5 above, any confirmation from Apple doc?
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.