When you uninstall an expansion...

Does it have a negative effect on your save? No corruption or anything like that?

If you are playing in the world that is included in that expansion you will not be able to load it. If not, objects from that expansion will be replaced with others.
From what I know, this is all it can happen.

Okay cool, i just gotta be careful when im on expansion worlds I guess. On that point I do have a couple questions.

1. Can you cancel certain aspects of an expansion? I like to play realistically so i dont wanna see vampires, ghouls, etc. Not even neighbors like that. Can I enjoy the expansion without specific details of it?

2. Seeing as how I get university life obhects while not at the uni and get an option to turn regular residential lots into apartments in sunset valley...does the same principle apply with something like the delicious indulgent bakery or any other dlc like late night? As in, accessing its contents while not being in its thematic world? I dont see it in my "Complete Sims 3" download so maybe it was bundled with another expansion?

If you have The Sims 3 Supernatural installed, you can disable almost all supernatural sims from the game settings. For the fool moon zombies you will need to set a single lunar phase (other than fool moon), and for the unicorn (it comes with Pets I believe) you may need a mod.
Deliciously Indulgent Bakery is Custom Content from the Store.

Cool thanks. i hear nraas master controller mod works for population control in cases where thats not supported. Very popular modder apparently

Thats great. I liked the pets EP before I heard more about it. By minimizing their spawns I may actually install it. Besides nothing lost since most EPS usually add a little extra anyway.