I just finished installing several oses but grub doesn't recognize opensuse. Can I add it back to the bootloader somehow?

Yes. http://www.howtogeek.com/196655/how-...ders-settings/

If your Opensuse doesn't boot, I guess you didn't install it's bootloader to the MBR. If you are using an MBR install rather than UEFI, you would need to let the primary system's bootloader know that you have installed another operating system. You haven't indicated which other systems you are using. Is it still Debian or Ubuntu? You would need to run either update-grub for the Ubuntu or for some other systems, the grub-mkconfig command from your primary system.

Which ones, and in which order? Was openSUSE the first?

Yes. How might differ according to which came last.

I installed bodhi, debian and mint, then opensuse. Everything was fine. Then I installed ubuntu and opensuse no longer appeared on the menu, and it had an ominous red circle next to it in gparted. So I reinstalled opensuse and everything was fine again. I should have left it alone, I know. But this was my quest to find the best distro, and bodhi was no longer beautiful, so I replaced it with mageia. After installing mageia, it queried me about what other operating systems I had, and since I didn't expect this, I didn't have the list of Oses and their partitions. So now I have mageia only. How can I get the others back, and why is this so different than previous experiences I've had with multibooting? Is it because I previously used .deb distros and these are .rpms? Or is it that they are uefis? Or was I just lucky? Any help will be greatly appreciated!

Why not "install" these distros as live boots from USB's?
You should be able to quickly decide on the definite rejects.

Failing that, install several distros as VM's on a acceptable distro.

Which Bodhi, Debian, Mint, openSUSE, Ubuntu and Mageia release versions did you install?

UEFI means I can't be of much help here. I have many multiboot systems, with all but one of the distros you listed, but none are UEFI.

I suggest you report your experience on the dev@ml.mageia.org mailing list and/or Mageia's dev forum. More than likely they'll be happy to know about the failure so that they can do something to prevent it from happening to others, and will instruct you how to do whatever is necessary to acquire a menu that includes all the others.

I guess the USB option makes sense, but I really wanted to use them normally, i.e. with everything I want installed; also, wouldn't a large distro, e.g. with KDE, be pretty slow? My internet connection is pretty flaky lately, so I think a vm wouldn't work so well. But yes, it may come to that, but with a TB disk, I really wanted to check out the strengths and weaknesses of each.

For a live distro,
If you have a reasonable amount of memory e.g. 2GB+ you should be able to load a lot of big packages.
1GB+ should be enough for KDE alone and similar.

You don't need an internet connection for a VM (unless you are accessing the internet).

Easy. Within your main OS, mount other OS, then edit /etc/grub.d/40_custom. Grab the lines starting 'Menuentry....' from the /mountedOS/boot/grub2/grub.cfg and paste that onto that custom file.

Then regenerate grub.cfg by issuing grub2-mkconfig -o /boot/grub2/grub.cfg

a.k.a "update-grub" on some distros...

But that is less universal, only works on Debian-based distros.