Please help....need to make something similar below. would you dowel the bottom tier as normal i.e 6 dowels? Im doing 3 tiers (4,6,8") Bottom tier buttercream rosettes and the other two ganached and then covered in fondant. Worried the chocolate is heavy for a buttercream bottom tier.

Also would you pipe the rosettes ontop and then place the next tier on? or would you leave the top, place tier on and pipe around the edge?

Since the tiers are relatively small, weight should not be such an issue....your dowels should be fine.  

As to stacking, I'd hold off piping the top until the tiers are stacked...it's hard tp judge how far over on the top of the lower tier to pipe and still have room to place the second tier.

Yeah.......what he ^^^ said :)