Can one create a multi-partition USB Stick with Mavericks, Yosemite, Snow Leopard installers?
Okay.. Apple have a perfectly workable website titled "Create a bootable installer for OS X" ( https://support.apple.com/en-us/HT201372 )

Which tells you what to do via the Terminal, once you have successfully downloaded Install OS X El Capitan.app, Install OS X Yosemite.app and Install OS X Mavericks.app.

Their instructions are as follows:

EDIT: note, added macOS Sierra, which works with the same method - but is not listed on their website

macOS Sierra

sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ macOS\ Sierra.app


OS X El Capitan

sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app


OS X Yosemite

sudo /Applications/Install\ OS\ X\ Yosemite.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ Yosemite.app


OS X Mavericks

sudo /Applications/Install\ OS\ X\ Mavericks.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ Mavericks.app


I was able to use these commands from the Terminal, onto a Disk Utility -made 5 partition USB stick. I verified it with option-down-during-boot (alt-down-during-boot) and both of them booted up fine.

I was unable to find an OS X Snow Leopard installer from my hard drive or elsewhere, so I'm considering this question answered.

Note: Mountain Lion installer app does not have "createinstallmedia" like Yosemite+Mavericks do, hence I didn't install that one on a USB stick - for that, one would have to use a secondary set of instructions for installing the installer to a USB stick via the usual way (Disk Utility, mounting, copying, copying some more..)