I used a combination of Great Stuff and a foam background. I sprayed a thin layer of GS, then pressed the foam background into it. Then used GS to fill the gaps and further shape the background. (Background was from a smaller tank and I figured why let it go to waste.)

It seems to have worked fine, but it is not frog-tested or cricket-tested yet. :-) My White's is still too small for that tank and it needs time to grow in. It is planted however.

Like others have said, the little gaps do seem like a pain. Your plans sounds fine to me.

In answer to your other question, I wouldn't let the foam extend into the soil or drainage layer. I'd be afraid of the styrofoam disintegrating into the water and being ingested or otherwise releasing something gross into their water or substrate. That's just me though. Have fun!