I am making a 3d despicable me minion cake for my nephew and they want him to be piped with butter cream but i want to know how to attach his arms. Would gum paste and fondant be to heavy and i would also be piping cutter cream on top or should i use rice crisps? 

Depends on how you want his arms. I wouldn't use gumpaste, too heavy. If you used fondant and just want the arms to fall down the sides you could just attach them using toothpicks or sucker sticks? If you want to shape them in a movement position you could maybe use wire covered in fondant, leave a little wire out the bottom to stick into the cake and shape them how you want before applying? Just a suggestion. :)

AI did arms on a 3D Pikachu. I made them from RKT and covered them in fondant. I originally had a skewer coming out the bottom of the arm, and slid it into the body so the arm was sticking out at an angle...this didn't work! The arm was too heavy for the cake and immediately started sagging and the hole in the cake just tore getting bigger and bigger.

So I changed it and decided to make the arms flat against the body. I put three skewers in, kind of perpendicular to the arm, and then slid them into the cake. I ended up with the arm flat against the cake, and it was very sturdy.

Good luck!

Thats some great suggestions thank you so much for replying! Do you think it would make the arms weaker if i put butter cream onto of the fondant?

Faradaye i think i will be happy with the arms flat on the body i was just afraid it would be to heavy  and break off, but the skewers would keep to in place. Thanks so much for the ideas!

AGood luck! Post a picture when you are done.