So, the iPhoto library (not to be confused with the "Photos library") takes almost 30 gigs on my 250 gb macbook air. I am curious, I've managed to upload the Photos library to the iCloud reducing it's size to mere 3 GB. Is there any way I can do the same trick with the iPhoto library? As far as I can understand, it's the location for my older photos, taken and uploaded before apple rolled the new photos library in?
No, you can’t. In order to do this, you have to migrate from iPhoto to Photos.
Photos contains the integration with iCloud storage that can offload to your iCloud account on demand to free up local space.
iPhoto is basically dead, isn’t it? How about exporting your pictures from iPhoto into iCloud Photos? Do it as a batch, a few at time, verifying upload before deleting the originals. You will likely lose any organization (albums, etc) unless you manually create albums and import into those, or just live with the automatically created events and albums iCloud Photos creates for you.