Quotas are assigned based on file ownership. If you need to retrieve data from backups for a user to a volume which is set with quotas, make sure you restore it to the final destination with the proper ownership. If you decide to copy it to the user's data folder using an administrative or backup operator's account, this account will become an owner, and files will no longer count towards user's space utilization. You also should be careful when deciding about the placement of spool folder and user profile folders. Since in both cases files generated are assigned user's ownership, they will count towards the space utilization limits.
Disk compression has no effect on calculating space utilization.
Sparse files use quotas equal to their total allocated size. Quotas are volume based, which prevents you from applying them on a folder level. Quotas cannot be assigned to groups, only to individual accounts. This might become a problem if you want to customize quota entries for members of different groups. You will have to either take ownership of the user's files, move them to a different volume, or delete them.