I am planning on ganaching for the crumb coat, and was wondering if you use the same ganache for your final layer. I want to pipe light yellow and lavendar flowers on top with butter cream, and that will work, right?

You can definitely use ganache as a final coat for your cake - check out this thread I posted for a fab tutorial on getting a super smooth and sharp finish with ganache.
http://cakecentral.com/modules.php?name=Forums&file=viewtopic&t=714218&postdays=0&postorder=asc&&start=0
Not too sure about the buttercream decos on ganache though.. I'd maybe google that topic a little to search for an answer. I've used fondant decos on my ganache.. they worked fine. HTH

You can definitely pipe buttercream on top of ganache. I just did one for a friend that's in my pics. I did a whipped ganache crumb coat, chilled it until firm and then poured on a ganache finishing coat. All of the deco. on the cake was with buttercream and I didn't have any problems at all.

I love using ganache as the final coat. Fondant accents work great, not sure about bc though. Here is a pic of a cake I made like this for my little sister.

http://cakecentral.com//gallery/1934669