I'm relatively new to cake decorating. If I am wanting to create a swirl pattern on a cake with marshmallow fondant, which type of icing should I use and what can I do to make it work properly? (buttercream vs. RI.... consistency, tips. etc...) This cake is similar to what I'm talking about.

I would use royal icing.

AI would use royal, medium peak with a small amount of piping gel added

I just use regular buttercream.  I have several scroll cakes in my gallery.  I just thin it enough so it can be piped smoothly without making your hand uncomfortable (squeezing too hard).

AIs use buttercream

did the shortening or butter in your buttercream effect the fondant at all?

I pipe buttercream onto fondant. Never had any problems with it

I would also use royal icing. An advantage is that if you mess anything up it can be picked off when dry! By the way, those curly cues are harder than they look...practice first on the sides of a cake pan, so you are working vertically like you will be on the cake. I think Wilton makes a set of plastic curly cues that you can use to emboss the fondant slightly - sometimes it's easier to pipe over a design that's already there than do it freehand. 

Oh, and I would use a number 2 tip, 3 at a push.

Yes I could too, but sometimes you get a greasy mark! RI doesn't tend to leave a trace.

Thanks for all the tips!

Thanks you all this has been very helpful!