AI am needing to make a smash cake. She is wanting an 8" cake 6" high. I don't want to put cardboard in his smash cake but I don't want the cake to collapse either...it's just buttercream, no fondant weighing it down...can I just add the extra 2" without a board? Beyond that...could I tack it 8" or would that be really pushing it? TIA

That is one heck of a smash cake!  What are the guests going to be eating while the baby it tearing all that up??  If there is no fondant and only buttercream I cannot see there being a problem with the height.  I prefer to add supports but in this situation you dont want the baby grabbing a dowel rod or board. 

AShe is a photographer and wanting the smash cake just for photos to put on his invite...I can't believe she wants it that big either. I have never done a smash cake this big..typically I prefer supports to!

I see!  Well hope it goes smoothly!  =-)