I am having a few problems in setting up RHEL7 alongside my other operating systems. I am trying to install it alongside Windows10, Linux Mint, and Linux Kali. The problems I'm having are:

I'm not familiar with the partitioning scheme RHEL7 does, so I had planned to just do automatic partitioning, and shrink the main partition later but the main partition setup by the automatic partitioning (lvm2 pv) is 325.84 GiB and is using 325.77 GiB of that space, so it's not possible to shrink it. It also shows this partition as always being mounted, no matter if I'm on Kali or Mint. Why is it already taking up almost the entire space it was allotted during the install, and why is it being mounted even when I'm not using RHEL?

Other problems include:

When booting up with the bootloader that came with RHEL7, I am unable to boot into either of my Linux partitions, only windows. When I try to boot into either Kali or Mint, I get the following errors:

error: can't find command 'linux'.
error: can't find command 'initrd'.

When booting up with the bootloader that came with Mint, Red Hat is not in the list, and I am only able to boot into either Mint or Windows 10. When attempting to boot into Kali from this bootloader, I get the following errors:

error: failure reading sector 0xfc from hd1'.
error: failure reading sector 0xe0 from 'hd1'.
error: failure reading sector 0x0 from 'hd1'.
error: failure reading sector 0xfc from 'hd1'.
error: failure reading sector 0xe0 from 'hd1'.
error: failure reading sector 0x0 from 'hd1'.
error: no such device: b644ccdc-7bef-4790-8427-bca75334b00c.
error: file '/boot/vmlinuz-4.0.0-kali1-amd64' not found.
unaligned pointer 0x9f2d3578
Aborted. Press any key to exit

Then it goes back to the last bootloader I booted from

When booting up with the bootloader that came with Kali, Red Hat is not in the list as well, but I am able to boot up with any of the other 3 operating systems without problems.

I think most (if not all) of the above errors could possibly be solved by using rEFInd, but before I try this, I want to address the issue about the disk space and lvm2 pv partition always being in use before I go any further because it's possible something went wrong in the install.