Bob,
Your layout construction idea sounds similar to Bill Darnaby's (Think Maumee Road!) I'd recommend you read his construction articles in MR. The first one is in the March 1995 MR, page 80. The techniques are more for a "fixed in place" layout rather than modular / sectional (IMHO).
When you ask about "essential foam working tools", are you talking about how to cut the foam to fit the space or tools used to shape it for scenery? For the basic cutting, we've used a table saw (outside if at all possible) with a plywood cutting blade. There is little kerf (proper term?) on these blades so the dust is held to a minimum. This makes quick work of a 4 x 8 sheet of foam.
Hope this helps!
Regards from Ohio,
Tim
For straaight cuts in 2" blue or pink foam all you have to do is score with a utility knife,and snap it off over the edge of a table. By score I mean set the utility knife to its longest setting and make one pass along your marked line. No Mess.