Re: "Trying to run iPad software update but not enough space".
It is wise to back-up everything securely, before you make space on your iPad:
How to back up your iPhone, iPad, and iPod touch - Apple Support
also:
How to: Archive or make copies of the information you store in iCloud - Apple Support
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Re: "If I delete photos and videos from my iPad can they be retrieved from my iCloud backup?"
It depends on how you set things up in iCloud ... eg: synced photos are not included in iCloud daily back-ups
if you delete photos from iPad that are in sync on iCloud, the synced copies are also deleted from iCloud
See: What does iCloud back up? - Apple Support for what a backup includes, how to use it, how it works etc.
NB: The full resolution photos may be in iCloud, with either high or low-res versions on the iPad.
You may need to take care, and be sure to keep your full resolution photos safe, before deleting photos.
See: How to: Manage your photo and video storage - Apple Support for tips
Apple support Video: How to Free up space on iPhone or iPad
https://www.youtube.com/watch?v=I2JujM_iLBM