I really like that terrorist figure - the parts work perfectly! I've always been nervous of boil and popping ball-joints because they never seem to pop out for me when I tried that in the past, but that was when I was first starting out as customizing so I might try again. I like the combination of the Clone Pilot thighs and the (possible

) Ki-Adi boots.
Darth Plagueis looks cool, I never bothered thinking what he looked like, so now that I've seen your version, that's how he looks in my head.
The foamcore base and walls look great.
When I was making my downed gunship diorama, I used blank printer paper to cover the seams (just glue it over the edges of the foamcore. That worked pretty well to hide the seams, but it took a bit of work so I didn't do it for all of the parts. If you're just dealing with right angles though, it shouldn't be too hard.
Also, for foamcore, I found that craft glue works really well (the one that I use begins with an "S" but I can't find my bottle - it's a blue and white bottle if that helps

).
Anyway, excellent work on that foamcore diorama, it'll look great for a gunfight
EDIT:
Here is a pic of what I mean:

You can see the edges of the foamcore where the sides connect to the bottom of the gunship - it's pretty rough.
Then if you look at the actual bottom of the gunship (at the angled joint towards the back-end of the gunship) you can see that I glued a piece of printer paper to hide the seam. Once you paint it, it's harder to notice and it's really easy to do on right angles. I should have done it on the rest of my downed gunship.
Hope this makes sense CHEWIE!