Must I always fit the admin userquota to the groupquota?
No. Maybe a short explanation of quotas might help you to understand this:
Userquota is the amount of data that a individual user migh store on the harddisk.
Groupquota (the quota of the website) is the amount of data which all users of the website might store on the harddisk.
The website quota is 200 MB and you have three users with a userquota of 100 MB each. You see that the userquota of all users is 300 MB. If user 1 stores 100 MB on the disk and user 2 stores 80 MB on the disk, the thirsd user can only store 20 MB on the disk even if he has a quota of 100 MB because he is restricted ny the group quota.