Can I run Yosemite after I install the Mojave firmware updates? I'm very hesitantly getting ready to "upgrade" from Yosemite to Mojave because I worry that it will break things I still need. So to give it a try I've decided to install Mojave on an external drive first.

Before I do that, I need to know if I can run my current Yosemite installation after I let the Mojave installer update the firmware. If I install Mojave on an external drive and suddenly can't boot my current Yosemite setup, I'll be hosed!

Can anyone confirm the firmware updates won't break Yosemite?
Yes, such a downgrade is possible.

Once shipped the firmware updates really change very little in terms of hardware capabilities. 

Those firmware updates in Mojave seem to be really mostly about adding support for APFS. Whether Appel included more substantial changes to existing hardware, like security fixes, are mostly unknown, due to the standard lack of transparency regarding update from Apple.

That means in personal testing I have never seen a device not booting the system it came with originally, despite numerous upgrades. The most extreme example being 10.13 on a MacBook Pro that shipped with 10.6.

The lack of problem reports for official Apple solutions (Apple: Use the Mac operating system that came with your Mac, or a compatible newer version) and other solutions (How to downgrade macOS from Mojave to an older version) indicate that the Mac itself should be able to run al those supported versions.

Caveat: 

That means it should be fine for an Apple computer and an Apple system software to mix and match as you desire. (Boot 10.9 one day, try out Mojave the other and then go back to Yosemite.) 

That doesn't mean that a possible security fix in one of the firmware updates (eg for how Thunderbolt operates) might not break interoperability for one of your other third party hardware peripherals. But even for these possibilities a problem report seems hard to find.