You can use a bit of water on a small brush as glue. Use can also make gumpaste glue by "melting" 1/2 tsp of gumpaste in 1 tsp hot water.
Since toothpicks are not edible, I wouldn't recommend using them to put flowers on your cake. You don't want it to snap off and end up in someone's mouth. Same thing with wires. I'm of the school that you don't insert wires into your cake.
Multi is right. You can use some water or gum glue. Other options include royal icing and melted chocolate. however, do NOT use melted chocolate if your cake is goign to be somewhere really hot, as it will soften and they will end up falling off.
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