Can you still download Yosemite? If not can I clean install Yosemite from a USB stick made for another machine? I've got several computers that I manage. Last week just before the release of El Capitan I upgraded one of them to Yosemite; I now want to upgrade the rest to Yosemite (the reason I'm not going straight to El Capitan is that a core piece of daily software I use doesn't work with it yet).

The other machines I want to upgrade are all currently running 10.8.x (not sure what version but it's the latest).

Is it possible to still download a copy of Yosemite to install? 

I've also got a copy of Yosemite saved to a USB stick that I used for the clean install of one of the computers. Could I just use this to clean install Yosemite onto the other computers? The first computer I used this on was an MBP late 2012 going from 10.9 to 10.10 while the other computers are a mixture of Mac Minis and MBPs going from 10.8 to 10.10. Would they need a different version of Yosemite to install or is it all the same version?
You should be able to download Yosemite from the App store if you login with an account that had previously acquired it.

If the USB stick was made with an App store copy of "Install OS X Yosemite.app", it will work on any hardware that existed prior to the release of the copy you downloaded.