Can I use S8+ battery on S8?

Technically you can. Nothing is going to stop you from installing a battery by taping it on to the back of the phone with a little bit of an air gap in the middle of it. This is how I charge my Galaxy S9 plus. I take my dad's phone for a minute and then charge up and then anchor to a charger for the entire day because my temperature sensor got screwed. I have noticed that over time the battery may swell if done in reverse. These batteries are older than modern phone batteries so they have no idea how many milliamp hours they hold only the phone does so if you have a 3500 milliamp battery and you want to benefit from the extra battery life you will have to charge it in a plus series phone every day and then take it out and put it into your Galaxy non plus model. At that point I would question why not just use the plus edition of the phone? You would only be able to charge it to 3000 milliamp hours instead of its fully rated capacity on the non plus variant alone, but if you're drawing the same load you would have drawn on the smaller battery the larger area of the larger battery will definitely help you maintain longer battery life to some extent at least. It's possible but largely not worth it aside from a couple of bursty loads on the battery like camera Flash or a sudden hard work load.

its does fit, what im worried its about functioning, if the voltages are the same it should work no problem