I did the same thing years ago. Between my own personal lessons learned over the years and trying to assist other people here, I now try to dissuade people from using multiple partitions especially since external drives today are so fast. There may be a few limited cases where multiple partitions may be necessary, but I think that should be extremely rare.
You can get older macOS installers from links within this Apple article which also includes instructions for creating a bootable macOS USB installer:
How to create a bootable installer for macOS - Apple Support
FYI, If you have more than one Mac (or more than one macOS boot drive on this Mac), then why not download & archive the older macOS installers you are interested in. Just create a folder and copy the macOS installer app from the Applications folder into the new folder (e.g. "Install macOS Mojave.app"), then use Disk Utility to create a compressed read-only .dmg archive file. Once you have the installer app, just about any Mac and macOS can create the bootable USB installer (at least more options than there are to download the macOS installers). Don't store a macOS installer in its app form as it may change over time without you knowing it even if you move the installer to another location.
I actually used an old hard drive pulled from an old computer and created multiple macOS bootable installer partitions on the single used hard drive so I have easy access to reinstall any OS of my choice when needed. I also keep a .dmg archive image of each installer app as well in case this old drive dies on me since I am running out of older working computers to be able to download and/or extract the macOS installers. It would really be nice if Apple provided access to all of these macOS installers outside of the App Store in a form that any OS could be used to create a bootable USB installer since not everyone has access to another compatible Mac. While Internet Recovery Mode is nice, it does not always work as intended plus it does not provide access to each OS compatible with a particular Mac.