If you use several forms on your page, can you then use the same folder for all forms you use on the page or must everyone have their own subdirectories with / Common / fbapp / storage ..etc?

Is any of these directories ok to delete after testing that everything works?

(hunting for the smallest possible size of the site)

Yes you can delete some of the files and or directories. It all depends on what your do with each form as far as features built into the Form Builder you use.