AI officially dislike with a passion MMF! Way to difficult to work with. If I were to buy one, what would you suggest? Also, my buttercream slides down when I cover with fondant creating massive bubbles. Should the BC underneath be really stiff? Mine was pretty thin since it was a crumb coat, but I could tell the minute I put my BC on that it was moving and bulging out on the corners and down! Gonna decorate the heck out of it to cover all the mistakes, but what should I do for next time?

We always use a ganache crumb coat under fondant as opposed to BC.  No sliding and gives a crisp edge.

We like Elite fondant.

Is it a butter based buttercream? It will melt if it's too warm and cause it to slide. Maybe try a shortening base or half butter half shortening mix. I use a crusting, shortening based buttercream under fondant and I sometimes have to spritz it with a bit of water to get the fondant to stick. Its that dry.

I also just starting adding glycerin to my mmf and it works really well. It gives it a softer, more pliable texture without being sticky. Just add a tbsp to your marshmallow before you add ps. 

AI typically use ganache and it's firm before covered with fondant-much easier! Or a butter based BC that's also firm before covering.

I don't care for Satin Ice but apparently it's a very popular brand. Fondarific and FondX are nice or I'll use a combo of modeling chocolate and fondant or Liz Marek's recipe has excellent reviews (you can google it-Liz Marek MMF from Artisian Cake Company) I keep meaning to try it.

Your location is something to consider, what weather conditions you have to deal with (extreme humidity will affect your results).

What recipe are you using? My first attempt was with marshmallow and powdered sugar homemade fondant, it was easy to work with but NOT for covering cakes! I could never get that elasticity that's so important-glycerine might make it salvageable or mixing it with a small amount of Wilton even might help.

AThe buttercream was 1 stick of butter and 1 1/2 cups of shortening, and stuck it in the fridge to firm up before uncovered in fondant. My MMF recipe is marshmallows, 1/2 cup crisp, 3 TB of water and ps, then I mix in a bit of Wilton into it. I live in colorado, so it is super dry here. I think I need to try a new fondant. I was so upset that my next cake in a coupledays I was just gonna put some flavored extract in some store bought Wilton! I just want a smooth cake! Lol

Colorado decorators here on CC hopefully will read your post and share what works for them.

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Great suggestion. Will look forward to trying that myself.

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Agreed. Marshmallow for decorations, commercial for covering.