Ok I hope I can remember all the questions.
1) The white foam..there is a pic above my post (y'all see the white and blue foam)
2) I just free handed them, larger ones for me, hubby, and oldest grandson. Smaller ones for the 3 middle grandkids and small one for my 6 month old grandson that's on the slide.
3) I but paint in gal, qt, or what ever I can get at lowes all year. You can't beat getting a $40. Gal of paint for $5.00 a lot of them even have primer in them, but I use any as long as its the right color....my gingerbread people are a little dark (my family says I burnt them) because my front yard is so big I have to paint alittle darker so the can see them good.
4) no water proofing, I use PVC , and cpvc pipes and gorilla glued them to the back
5) I found on here everybody uses expand foam for all the icing.....that's too hard, hard to paint and messy. I got every tube of white and clear silicone caulk Lowe' s had (at a dollar each discount) which I was going to use around house. And it is perfect no painting and you can wiggle it as your putting it on to look like frosting I hope this helped. This was my fist attempted at this, then I found out that it looked really good..