You can use a single one piece .x file model and skin in FPSC EA now, and have it the full 40 x 40 tile size if you wish, however you would be advised to wait for V1 as the inclusion of Physics will allow you to walk over your Terrain (Not in EA). It will not require that you have any additional floor segments under it in order to walk over it. From the tests that I did a while back a Terrain used as an .x file entity should not unduely impact on FPS. See my post somewhere here - do a search and look for it - it contains a link to a video showing FPSC and Terrain in action for those who missed it. Anyone know where it is now? I dont - lost the thread.
You could of course spend the time while you wait actually making your Terrains and Skins so when its released you are ready to go.
Some tutorial type tips :
The best way to make your Terrain to fit is - in the FPSC Editor take a screen shot of your whole level so that you can see where your World object construction is and where you need the Terrain to avoid this, with a little ingenuity you can use this screen shot as guide to make your Terrain - making some areas completely flat to avoid your buildings and alike. Mounatins you can keep to the perimiters or whatever your preference is.
In any case once made you can adjust the vertical position of your Terrain entity in its script file so that its positioned just right, allowing you to also place ground segments as flat paths the top of being slightly above the Terrain surface and over it if you wish.
Creating a one piece Terrain designed specifically to fit in with a map layout can be a time consuming task...but it can be done, placing, moving or selecting it correctly in the Editor is a little tricky when its full of world objects as for a 40 x 40 tile Terrain you have to scale it so it fits exactly to the 40 x 40 area then in Editor - the entity Terrain is loaded and it has to be placed in the exact centre tile of your map (because that where it exists to FPSC) at the level you want it - I suggest build your map first to completion and place your Terrain last when you are happy with everything else - a little effort and patience is needed and you will be well rewarded.
I would suggest that anyone interested in Terrain should copy this thread and keep the info for future reference...
Hope that helps.