You can reapply for scholarships I think. I haven't tested this but I just assumed so as the option is available on letterboxes etc when you are rotating between the households.
I think it's probably better to not have any rotated away from households actually enrolled in uni as they will only do averagely well and you can not replay a semester or a degree once it is completed. So unless you are ok with not getting an GPA A+ then only have actively played sims in uni.
If you are playing one sim through multiple semesters it is better/easier to enroll when the prompt comes as I chose no and thought I'd enroll later but the game play is really funky with that option. I ended up having to move my sim into a normal residential lot as I just couldn't get moving back into a dorm when re-enrolling, which was really buggy.
So for me I've found it best to only have the one household enrolled at uni and play them until the end of a semester then move them back to their old home and then rotate to the next household. I have aging off as I particularly dislike my game and worlds changing in any way unless I have made the change happen myself. So I micromanage every aspect of my game. lol I can't stand any sort of progression of any sim, townie or not. When I want sims to age or marry etc I will either make the changes through Manage Worlds and CAS with cheats or play them briefly to organize the townies how I want them mark them as unplayed again.
To regenerate townies you can make some yourself in CAS and then drop them onto an empty lot then evict them. That way they will go into the pool of NPC's that are 'Not In The World' and the game will use them for the various NPC roles, vendors, roommates etc. or you can do the same but instead of making them just download sims of the Gallery.
If you have the 'Fill Empty Homes' checked then the game will generate townies/NPC's to move into those houses which is another way to regen your worlds.
