Installing new distro into separate preexisting partition while current distro is running
 

I have several partitions on my main machine. One is a 50 GB ext4 partition meant for testing distros out directly on the machine without a VM. Is it possible to install a new distro there while my main distro (currently LMDE) is running? If so, how would I do that? I want to avoid creating another installation USB.

You have to boot the machine to run an install to bare metal. I think that rules out running an installation when the system is already booted.

Of course, I've not tried this, and I never will.

Just my thoughts.

Maybe it is possible if you run the install in chrooted environment. Have never tried it either.

Debian can be install from a running system: https://wiki.debian.org/Debootstrap
Another option is to boot install iso from the hard drive using grub bootloader, requires a reboot.
Knowing what distro you want to install would help.

Why ?
Ventoys let you add/remove as many ISO's as your USB's capacity permits (copying, you don't have to install anything).
I use Yumi, it takes a bit more work, but it's still very simple.

Fedora.

Even though it may be possible with some or even all distributions, I think it is far more work than just creating an install USB and installing the normal way.

I find this a poor excuse and not enough reason for going through all the hassle mentioned above.

You can boot and install Fedora from LMDE by putting the iso on the LMDE but you will obviously need to reboot but not need a USB.

If you don't want to create an installation USB, you can put the Fedora iso on the partition on which you have LMDE installed, put an entry in the grub.cfg file of LMDE and then reboot and select Fedora from the boot menu to install it. The menuentry below worked to boot the specific version of Fedora listed so you can put this in your grub.cfg of LMDE with necessary modification. The change include putting the correct drive/partition on the loopback loop line (hd0,gpt5), the exact name of the iso on that line, the correct CDLABEL name and the correct iso name at the end of the linux line. If you have a different version of Fedora, I don't know that it will work.