There are no documents that cover this subject put out by Microfocus as each client and corporation have different needs and requirements for maintaining backups and availability and therefor the responsibility of the individuals clients/corporations. Although there are no recommended documents that cover archival, what is generally suggested is when just attempting to free up space, to backup the repository and the project schemas for those projects not actively in use and then move those backups to a secured storage location off of of the application server to both free up space and make for easy accessibility in case the project should be needed in the future (just make sure to keep the project upgraded to the current version of ALM in use to avoid restoration issues in the future).
In your case since you have all projects active this wouldn't be a viable option for you. What you could do is to back up the repository and the schemas (do this with the projects deactivated to insure that changes don't occur during the backup process), you could then copy these backups to secure storage and then activate the project and go in and remove things not needed from prior points in time that could be using large storage (such as attachments, outdated requirements and tests/test sets, run data, etc that could be consuming large amounts of data) and then if needed you would always have these available to restore from the backups (such as creating a new project from the back ups and then dragging the needed information back into the current project), but note that any linkage and coverages will have been lost when removing the data and will need to be recreated when/if the data is added back in.
This last would be the only way that I can think to address this issue with your current situation and not adding more storage to the system. Also know that every project that is added and active adds more overhead to the system in terms of CPU consumed. threads opened, and both physical RAM and hard disc storage to the system, so it would be easier on the system to have as few projects as necessary to ease overhead and increase performance on the system.
Hope this helps,