Just wondering how much buttercream you would have mix with Whimsical bakehouse buttercream to make it crust? Probably would lose the fluffy texture...?

A lot. And you are right, you would. To get it to crust it is not a matter of adding a crusting buttercream, but the ratio of sugar to fat. I know this one I think is a little close in ratio. I can't remember the weight of the shortening added. Plus melting the PS makes a difference also.

Maybe you could try adding some meringue powder. But that will change the flavor, so experiment with a test cake first.