[TYPO3] Typo3 limits ?

[tapio]

> Is it possible to create some groups inside other groups ? 

a group can include other groups, users can be set to several groups

> Is it possible to have a calendar and a file manager restricted to a
> certain group ?

yes - access control comes to table field level (some fields can't be 
taken off with default setting - with you own setting possible to get 
also those fields controlled).