Hi!
1. In this case, I would rather suggest using the inclusion options, instead of excluding all other categories. So, make sure to clear all exclusions, and use the include posts by taxonomies option: Tutorial – Include posts by categories
2. You are looking for this option here: https://i.imgur.com/U91lyoy.png
3. Under the Theme -> Isotopic results panel: https://i.imgur.com/cyBkmcm.png
4. Try loading the ‘Plain Round isotopic’ theme, it should be very similar: https://i.imgur.com/0P2Kra4.png
Then you can customize size/margins as you desire.
I hope this helps.