Can I delete photos from my Mac's library after I upload them to my iCloud Photo Library beta?

Do you mean you you want to upload photos from a "Photos" library on your Mac to iCloud Photo Library beta? All Cloud Photo Library beta on all devices will sync with iCloud. Deleting a photo on any device from Cloud Photo Library beta will delete it from iCloud and all other devices. But you can save space on your Mac and other devices by using the "Optimize Storage " option. Then only smaller preview images will be kept, but the full version will be in iCloud. See the Preview - it is explained there:

Apple - OS X - Photos Preview

It doesn't make it clear that it "optimizes" what used to be our locally stored iPhoto Library on our Macs.

My iPhoto Library takes up 250GB. 1/3 of that is "Previews" and "Thumbnails".

It doesn't make it clear that it "optimizes" what used to be our locally stored iPhoto Library on our Macs.

That is an option, that you have to enable. That is meant by this paragraph: Apple - OS X - Photos Preview

Fill your library, not your device.

iCloud Photo Library can help you make the most of the space on your devices. When you choose "Optimize Mac Storage", it stores all your full-resolution photos and videos in iCloud in their original formats, while keeping storage-saving versions on each device. When you make an edit, the original image will download automatically. You get 5GB of free storage in iCloud — and as your library grows, you have the option to choose a plan for up to 1TB

The Thumbnails and Previews are locally needed, the "Masters" can be in iCloud.