Can my Mac save photos from iCloud Photo Library, but not upload to it? Usually I take photos with my iPhone. On my Mac I have a more extensive photo library, and the masters are referenced from an external hard drive. 

iCloud Photo Library is enabled on my iPhone, but not my Mac, because I don't want all of the full-resolution DSLR photos on my Mac to fill up my iCloud space. However, now when I take photos with my iPhone, it seems the Mac doesn't download them because iCloud Photo Library is off.

Can I somehow instruct my Mac to download photos &amp; videos from iCloud, but not upload to it? (I see that Photo Stream is a separate option, but I believe it doesn't support videos. It might work to simply not open Photos while my external drive with the large library is connected, so it doesn't upload them, but continues to download in the background — but I'm not really sure that will work.)
Photos on the Mac can have two libraries. You could declare a new library as the "system" library that syncs with iCloud to get a downloaded copy of those images. That is the best you can do if you don't want all the Mac photos to upload to iCloud (short of using another program than Photos app).

If that meets your needs, you could hold option (alt) when you start the app to create a new library and/or switch between libraries. As you correctly observe, everything in the system identified library syncs up to iCloud. This syncing happens both ways in the background whether photos app is running or not. 

I don't recommend two libraries in Photos for most people - multiple libraries sounds good to many, many people and after a while, the majority of them regret making two (or more) libraries. You could be the exception, but I'd consider trying to make one library work for you if at all possible. I'm quite pleased with letting iOS and my MacBook compress images as needed and my Mac Pro keep full size versions of everything. A 150 GB library takes about half that space on my devices that have less than 300 GB of storage and I've never once missed the quality of an image.