Do I understand you correctly? Currently you are not yet using iCloud Photos, but are syncing photos manually to your devices by connecting them to a USB port on Mac, and downloading new photos and videos from your iPad and iPhone to your computers?
This way it will be natural to have different photos on all your devices. The manual syncing is only one-way. Photos and videos you are uploading from your mac to your iPhone or iPad using a cable are special, optimized versions for viewing on the device. They are not saved to the camera roll, but stored in a separate "Albums from My Mac" section in Photos and cannot be downloaded back to the Mac. The manual syncing is just one-way.
If you want to have the same photos on all your devices, you have to keep the photos you want to sync across all devices on one of your Macs as a master copy, copy them from there to your other macs, and sync them from there to all your mobile devices, iPad, iPhone, iPod Touch ..
Only with iCloud Photos it will be much easier to keep the Photos Libraries identical across all devices. iCloudPhotos is my main reason for using the Photos.app an all my devices to organize my photos and videos. my photoa and videos are always perfectly in sync, independent of the system version on my Macs or mobile devices, and independent of the device model.
When you enable iCloud Photos on all your devices, the photos and videos and albums and folders from all device will be uploaded to iCloud Photos Library, and merged in iCloud to avoid duplicates. From there, they will sync back to all your devices, to keep the libraries on all devices identical. You will always have the same items on all devices. Whenever you work with your Photos, adjust them delete a photo, import new photos, the changes will be updated across all your devices. Your photos will be in full resolution on all devices and in iCloud.
If you want to save storage on one of your mobile devices, enable "Optimize Storage" in the Settings > AppleID > iCloud > Photos. The iCloud will remove some of the high resolution originals from that device, if you need to free some storage. But the drawback will be, that you will need an internet connection to be able to access the photos in iCloud to work with them. I avoid this setting and are keeping no more photos in iCloud than will fit onto the device with the least storage.
I strongly recommend iCloud Photos, if you want to keep the photos libraries on all devices in sync.
But before you enable iCloud Photo on your mobile devices, you have to ensure, that all photos you want to save are either in Photos on one of your Macs or saved to Photos on one of your mobile devices - not only in the "Albums from My Mac". Turning on iCloud Photos will delete all synced photos in "Albums from My Mac" from your iPhone or iPad. You need to save any photo from "Albums from My Mac", that you have not yet saved on your Mac, before you enable icloud Photos, or these photos will be gone. Air Drop the missing photos, or Mail them to you, or try to edit them and add adjustments. That will save them to Photos as regular photos.
