Let's take a quick look at a step-by-step guide detailing how to turn an SSD into a boot drive for your new or existing computer.

Use Macrium Reflect to make a disk image on an external drive. Use same to make a bootable copy that allows you to copy the disk image to the new SSD. I just did it, refreshingly easy.

It’s possible you could force HFS+ on a boot SSD with High Sierra with some Terminal monkeying around, but I can’t see that would end well, and I’m not sure it offers any advantages.