For the bootloader questions: You just have to go to your bios (spam a function key during start up, which function key depends on manufacturer) and change the boot order. The order of things which happen when you startup your machine is:
your bios starts up
your bios selects the highest priority bootloader you have (you want this to be grub)
you can choose which OS to open in grub, if you don’t choose, it goes with whatever is set to be default in the grub config. If you haven’t edited the grub config, I think this would either be the first installed OS or the first alphabetically
grub runs the startup sequence for the chosen OS
For the other questions: You might have to manually choose what to mount where. For each distro, you will want to mount a boot partition (your grub partition), a swap if your ram is low (make all your distros share the same swap partition), and a unique home partition.
You might also want to mount a shared files partition. These would be files you want stored locally that you can access from all the distros. Don’t mount this in the install process, instead mount it after you install from whatever file manager you use on each distro. Make a ~/shared folder and mount it to that.