I've never done a stacked cake but recently started working with Fondant and going to make a ninja turtle cake for a friend.

The bottom layer will be half a sheet cake (or full undecided) with a 9in circle cake (only one) as the sewer in the middle and then a 1 litre bowl cake as the ninja turtle head ontop of that with the sewer lit built out of rice crispy covered fondant attached to the 1litre bowl head. 

I am going to put dowel rods in between the layers to futher support this cake, as I learned from reading some of the forums here but do I need to also put cake boards in between or will it be okay??? The forum i printed just stated to use a cake board as an outline/imprint where to place the dowels

Really appreciate the advice and the help!

You should use cake boards, otherwise, the dowels just go through the cake layers. The dowels only work to distribute the weight on a cake board. I would put one under the 9" cake (it helps to ice this cake first, then put it on the sheet cake). Depending on how heavy the head is going to be, you probably would not need a board under that one, since it's basically going to be a two layer cake then. Good luck!

Thank you! I've heard of cake boards absorbing moisture and making the cakes sink. Do you recommend a certain kind of cake board to prevent this from happening?

Thanks again for the help 

I've heard of cake boards becoming moist and the cakes sinking.. do you reccomend a certain board or brand that would prevent this from happening?? Thanks again for the help :)

Also, Its a marble cake and she would like some kind of chocolate filling. I was thinking chocolate buttercream even though vanilla buttercream is my crumbcoat since its shelf stable. Any other suggestions?

I just use cardboard cake boards, and have never had a problem. A chocolate buttercream or ganache filling would go well with marble cake. I use regular buttercream for my crumb coat, unless the cake itself is also going to be covered in chocolate buttercream. Good luck!

Thanks so much!