I can tell you what NOT to use. I used Starburst type candies to make little cut-out flowers for a recent cake because I didn't feel like making a whole batch of MMF for the little bit of flowers I needed. It worked fine for the flowers that I stuck on the side of the cake except for the ones where I mixed the pink and purple together to get a different color. Only those, no other colors, looked like they were melting and sliding down the side of the cake after sitting for a few hours, yet they were still hard. I used the same candies to make stand up letters for the name on the top and some of them stood up fine and but a few kept drooping so I had to back them with several toothpicks. Next time I'll just make the fondant. From what I have read on CC I would go with Mikail's suggestion of using royal icing. I think it would be easier than the gumpaste.