Design of small tables: clustered index or heap
As for subtables, I have a rule of always adding a primary key. In this case I would go with a new autoincrement column as a PK, as the index will be pretty narrow.

Secondly, I would consider propagating information like IsActive from UserSituation to User with a trigger (I assume user can only have one situation at the given moment). This may eliminate the need for one JOIN in most cases. Not sure it will work with UserCategory (it will not if it's a true 1:N relation).

As for clustered index on UserId, it sounds like a good idea on table User and a not-so-good idea on subtables. But I have to admit it is not my field of expertise.