Hi, victorian. Welcome! I’m not sure if you have Rose’s The Cake Bible, but I know the rosettes take a lot of buttercream. I can’t offer specifics since I don’t know the sizes of your tiers… I’m guessing, but I’d think at LEAST double Rose’s recommended frosting guidelines in the Wedding Cake section, plus another batch? I’m not sure what kind of frosting you are using, but I’ve only made Mouselline with butter. Since you are indoors, I don’t think the shortening is necessary. Some people use it because it is more stable at higher temperatures than butter; however, you have a nice, temperature controlled environment to work with and you should be A.O.K. with butter! Lucky you!