If you are just soaping for yourself and close friends, I wouldn't bother with any of the "preservatives" you mentioned. Just keep your finished soap in a cool, dry dark location until you are ready to use them and they will be fine. "Preservatives" come into play when you are considering selling your soaps. At that point you need to accommodate storage in less than ideal conditions, which can promote rancidity (DOS). Needless to say, soaps covered with orange spots don't sell well, and so some method of delaying the onset of rancidity is desired. Depending on how "natural" you want your product to be, this may be rosemary extract, tocopherol, BHT, etc. Some methods are more effective than others, so you will want to test with your soap blend to determine the optimum solution when you reach this point. Good luck!