.. If I was to make my own fondant.. would it be possible to use marshmallow fluff instead of melting the marshmallows?

Yes, there's a recipe that specifies marshmallow fluff:

http://cakecentral.com/cake_recipe-2148-0-Easy-Marshmallow-Fondant.html

HTH

In regards to making fondant, I am planning on making my first homemade batch of MMF and was wondering do you use the dough attachment the whole time or do you start with a paddle and then switch to a dough attachment when it starts to get thick? TIA!

You're going to love MMF! I just started making it in my kitchenaid mixer and it works beautifully! What you do is coat all bowls and utensils and attachments for the mixer with shortening (VERY IMPORTANT). Melt the marshmallows in intervals with your water. Pour half the powdered sugar into the mixer bowl and push it to the sides forming a dip to pour the MM into. Pour the melted MM and mix with the paddle first. Then clean off the paddle and switch to the dough hook and add rest of PS. If you leave the top of the mixer unlocked it will start to "jump" when the mixture is ready. Take it out and scrape remaining on top of MMF. You will now need to knead it a bit more with your hands and some shortening. Good luck! It's so fun to work with, messy, but FUN!