I'm new at using ganache, can I ice a cake wih buttercream and then pour choc ganache on top, letter it run down sides? Will this melt the buttercream? Thanks for any help.

I'm new to baking, but yes you can put ganache over BC crumb coat. The ganache can be cool if you decide to dribble it over cake. Or you can let the ganache set longer or whip it and it will go on more like a BC.