Hey gijii,
Welcome to the forum! And welcome to hackintoshing.
Congratulations on your system choice - I don't have my board anymore, but money-to-performance I've always thought it's been great!
To answer your questions:
1) The guide will generally work for most MacOS versions. I would suggest you stick to High Sierra 10.13.6 - it will give you compatibility with your NVidia graphics card when you get one, whilst Mojave will not.
2) Clover will always give you options to boot in either OS no problem. It detects them and boots into them just fine in my experience. I would strongly suggest though, that you disconnect all drives while installing MacOS to make sure no bootloader mix-ups and mis-haps.
I would suggest you read the guide and then read it again. And then again. Also - skim through the thread - there's plenty of useful information which, sadly, is not curated in the OP. However - what is there, is SSDTs, exemplary config.plists and useful instructions that (mostly) work!
Last but not least - I'd suggest you go for at least 250GB SSD for your MacOS install. I thought 120 will do me just fine, however, once set up you tend to find yourself using the OS most of the time(if not all!) and the disk space melts!
Post any questions you may have and I shall try and answer as best as I can.
Happy hacking!
littlegreen