Is a bootable USB drive with Yosemite OK for different macs? This might be obvious, but probably worth a question: I took my MBP (Early 2011 15"), downloaded Yosemite and created a bootable USB device from which I later clean-installed the OS.

Can I use this same USB device with a different MBP (i.e., Late 2011 13"), or do I have to download the installer again from that macbook app store?

I got worried for a second thinking that it might need different drivers, or something like that. Second MBP does not even belong to me, so it will be used with a different Apple ID (i.e., different from the one through which I got my Yosemite).
The Yosemite Installer package contains all the divers for all supported models.

You are good to go for multiple uses on multiple systems.