I've made marshmallow fondant a few times and I really like it. Tastes pretty good and is easy to make. Here's a link to the recipe and instructions that I use:
The only thing I do differently is I grease the glass bowl with Crisco that I melt the marshmallows in and then mix as much of the powdered sugar into the melted marshmallows in the same bowl as I can using a rubber spatula (also greased). Then I grease my really big metal mixing bowl and put some powdered sugar in it and then put the marshmallow mixture in the bowl and keep mixing by kneading the mixture. It makes less mess for me. I followed the exact instructions the first time and I had powdered sugar EVERYWHERE.
For the swirly designs (scrolls) you could probably find a stencil but I just freehand them. You can practice them on paper holding a pen/pencil/marker the same way you would hold the icing bag and then practice with icing on waxed paper before you actually try them on the cake. You could also make then with the fondant by rolling out long strings on the fondant and then making them into the swirls and putting them on the cake. I'm not sure which way would be easier for you, I'd prefer the frosting myself.
You didn't mention if you were covering the cake in the marshmallow fondant but if you are I would suggest looking up some youtube videos that show you how to cover the cake. I've only done it once so I can't help a lot on that one. If you're covereing the cake with regular frosting (buttercream, etc.) there are several methods for getting it smooth and again I'd suggest watching some youtube videos.
You'll also want to research and determine what you are going to use to support the cake tiers (SPS, wooden dowels, etc.) I've only made 2-tiered cakes so I can't help much on that either but there are lots of people on CC that are very experienced with tiered cakes.
I would definitely suggest making a practice cake before you make the actual wedding cake so you can see what works for you and what doesn't and adjust things as needed. Hope this helps a little and good luck on your wedding cake!
Yes, make a practise cake first. Not to be rude but you will probably reconsider doing your cake after that! There is a lot that goes into making a fondant covered wedding cake like the internal supports and damming (sp?) your fillings etc.
I think this particular cake they free handed the swirls. There are kits that have the same type of swirl though. I have a kit that my Mom bought when she did wedding cakes about 30 years ago. I know they are Wilton brand but not sure if you can still buy them now.
Again you could practise the swirls and might be surprise at how well they turn out.
Good luck and congrats on the wedding.