Storing pictures in iCould that are not in a Photo library

How do I store pictures in iCould that are not in a Photo library, but are in my Pictures folder? I have an iMac with 500 GB of storage with 179.9 GB available, and my Picture folder size is 62.37 GB.

However my MacBook Air has only has 250 GB of storage with with 120 GB available, and my pictures folder has 14.2 GB on disk which is only Photos Library.photoslibrary.

If the pictures that are in my iMac Pictures folder are downloaded to my MacBook Air, I will use up half of my available storage, and storage space might become tight.

Can I just keep thumbnails or small versions of Pictures on my Macs that are in my Pictures folder and only download full size versions from iCloud when required? On both my iMac and MacBook Air in Photos preferences I have under iCloud Photo Library, Optimize Mac Storage checked, which stores full-resolution photos and videos in iCloud. Originals are stored on my iMac "if I have enough storage". (What is enough storage?)