I’m try to answer based on what question i found there.
- fdisk definitely work if you installing using bios, and gdisk for uefi.
partition tables can expect 1MiB offset for the begin of the first partition, means 2048 sectors for 512b or 256 sectors for 4kb disks.
so when you fdisk there, the first sector should show the default first sector 256 which is fine and good to use.
- actually 20gb is enough if we can manage what data on root partition, as example sometimes we install 2-3 template at once it could cause template install fails because not enough space in dom0, and some users here experience it (back then when everyone was fail installing kali template), and i think you might fail too, since 20gb is not enough for installing 4 default template, except you install 1 by 1, and delete previous data.
If you want to give btrfs a try it also good, everything is work out of the box. for the layout you can find here just ignore 1-2 thing there in the drive section.