Please help,  I need to find out if i can frost a cake with butter cream frosting and then use cutout fondant shapes to decorate it?  Will the butter cream make the cutouts soft and cause them to run down the cake?

I don't see why it would be a problem. Ive had plenty of buttercream cakes with small fondant accents. i would recommend putting some buttercream on the back of your fondant shapes to make sure they stick, especially if its a crusting buttercream. 

Yes, you can use fondant cutout on b/c frosting. Let the cutouts set up just a little . Then apply after your b/c crusts . I used g/p showflakes on my sisters cake(buttercream iced) froze it and the snowflakes melted into the icing on the sides of the cake when it thawed . The ones on top did not. You can freeze b/c and or fondant cakes already decorated etc. You have to wrap them in a box, twice seran wrapped and then heavy foil wrapped and then take out 24 hrs. before serving and put in fridge and then right before serving, take out of box. I didn,t do that and my decos melted. It was for my sister, she didn,t care. lol!! hth  My instructions comes from BlakesCakes on this site. works great!!!!

And you don't need to wait for the bc to crust.  Just slap those fondant decos on the cake.  done.