The "rule" is not to exceed 1 TBSP of oils/butters per lb of mp base, otherwise the soap can get too soft and take a long time to set up.
In my opinion and the opinion of lots of other soapmakers, adding oils or butters to mp base is just a waste of ingredients. The butters & oils aren't saponified, so they're just going to get washed right down the drain when using it. Plus you lose bubbles and lather when adding them. I'd save them to make a nice stay on product, like a balm, solid lotion or a whipped butter.
As for adding dry ingredients like powders or oatmeal, really it matters what the ingredient is. If it's ground oatmeal, I wouldn't exceed 2 TBSP per lb of base, otherwise it can get mushy. For cocoa powder, I wouldn't go much above 1 tsp per lb of base, otherwise the soap can leak out the cocoa, which when being used looks kind of gross to see brown stuff running down your shower or on your washcloth. Spices like cinnamon or ginger, I'd use about 1/2 tsp per lb of base because they can be really irritating to the skin.
