SMBC question

LOL, it's a matter of taste - I only make SMBC and my business does very well - but I do get the occasional person that thinks its too buttery or wants something way more sweet.

People do make SMBC with shortening.  You just have to realize is that instead of using a tsp of vanilla flavor, you can use 1/3 cup of vanilla bean paste, or instead of using a 1/2 tsp of "strawberry extract", you can use 1 cup or real strawberry puree and it's divine.  It has an advantage over regular icing because unlike regular icing, you can hide the shortening flavor with real fruit purees, caramel, Nutella, nuts, booze, you name it - and you can use a cup or more and the consistency of your buttercream will not change at all.  

I would stay away from margerine and just keep it simple.

If you make it and it needs a little something, try adding in a few cups of ABC.  It makes the consistency even more whippy and adds sugar without making it gritty or too sweet.

Good luck!

Think about it like mixing fruit into your butter. If it's cold, it would be hard to the touch, at room temp it would be soft. I would steer clear of adding chunky fruits to the BC directly under your fondant as you don't want a bumpy finish under your fondant. I would suggest using the fruity BC as the filling and covering your tiers with the flavored SMBC. If you must have the fruity SMBC under the fondant, be sure to puree it well, and if possible strain out the big chunks of fruit.