Well, I know a lot of people have taken to leaving the dowels a bit higher, but actually they should be flush or a bit lower. This is because it makes it easier for there to be a slip and slide issue when a cake is just sitting on the tiny edges of dowels. Your dowels must always be exactly the same height. Your cakes should be perfectly level. If you think about it this boarded cake is sitting with only the little dowels supporting it whereas when they are flush or just slightly lower, the board is fully supported.
Regardless, if the icing is set and the dowels are adequate, the cake is supported by the cake, the dowels and the board.
I have yet to have a cake stick to another cake, it just has never happened. Not being a fan of coconut, I don't use it in between but of course it works. So will cookies crumbs, cake crumbs, shaved chocolate anything like that. I tend to use icing sugar as this blends into the icing easily and still provides a barrier.
If you are stacking a cake that has a non crusting form of icing or topping, like a whipped cream topped cake, you will need to have a substantial barrier like coconut as the nature of whipped cream is to be not firm. Personally I don't stack cakes with these toppings.
I think that it is always good to do a dry run or experimental cake, when you are trying something new. Even a small version will give you a better idea as to how things will work out. It is beyond my understanding why so many folks take on orders for something like this and do their experimenting on a customer. Heeehee, one of my pet peeves, those posts or emails that start with: "Help, I am doing my first wedding cake for a paying customer, she wanted a stacked cake, I made it and it isn't working, it is due tomorrow, what do I do, help?" What is that line about, "Proper planning prevents poor performance", haha!
If you have never stacked a cake, make up a 4 inch and a 6 inch cake and experiment. Even only two tiers will give you the experience.
Hugs Squirrelly Cakes