I am new to cake decorating and want to attempt this cake. I am wondering how I best support the balloons? I think I will make balloons from foam balls covered in fondant. Will that be light enough to be supported only by wire straight into cake? Or do I need to put a straw into cake, then wire into straw?! And what gauge wire? Your help is appreciated.

I would use skewers, they are then food safe and you can insert straight in to the cake.

I've also heard of people wrapping the wire in cling wrap to keep the wire off the cake.