I was wondering can I can I cover fondont or molding chocolate over swiss meringue buttercream

Yes, you can. Cover the cake with the buttercream (getting your edges nice and straight), pop it into the fridge for 10 minutes or so until the buttercream is firm then cover with fondant. Have fun!
Cris.

Yup! I use SMBC on all of my cakes (mostly fondant covered)... just make sure the icing is firm... I think it's easier to cover a SMBC cake than when I used America Buttercream, the firm surface of the cold icing lets really apply pressure to smooth your fondant/ modeling chocolate!

I made this icing for the first time this past weekend but it turned out really soft. Can somebody tell me what I might have done wrong?

If you are used to American buttercream, this is much softer by comparison. If you are sure your measurements were correct, it could have been that you didn't let the meringue cool enough before adding butter (the bowl should be cool to the touch)... usually it's salvageable... just let the mixture cool a bit and remix. Sometimes if I need to cool the bowl faster, I pack bags of frozen veggies around the bowl!

Am going to make a 3D cake Tow Mater and I always use whipped cream frosting on all my cake my I want to use Fondont or molding chocolate and I know you have to use buttercream for that I think buttercream is really sweet and I made the swiss buttercream and it was perfect it wasnt sweet at all.