Hmmm, I'll try tacklign some things...
-Fleet Riekean... Not sure if you're wanting Riekean in the Madine outfit or not Anton but if it were me I'd dig up a Captain Antilles figure and pop the Riekean head onto that uniform since, to me, that's the "Fleet" uniform plus the insignia's match up of course... I consider the Madine duds a whole other branch of the military... Not to get into technicalities or anything, it's just how I explain the differences in duds.
-Hoth Madine... On the Major Derlin body, I haven't popped one yet but I'm 99% sure that the neck peg is separate and pops out of the body. Why, I have not clue as it's somehow more complex without adding squat to the figure but Hasbro's done this with a couple figures int he last year or so.
Like CHEWIE mentioned, if you have a dremel that's your best bet... Without having them both popped apart completely it's tough for me to tell ya if a Madine noggin' would pop into the Derlin body and retain articulation and move fluidly, and still look right too (IE: would the neck length of Derlin be long enough to fit on Derlin's body and look right?). What I'd do is cut the Madine head off and hollow it so it fit the ball on the Derlin neck, however this gets touchy and could be fairly easy to mess up if you're not "comfortable" with using a Dremel tool... Or fitting a regular neck/head figure to the new "ball/socket" necks.
That's some advice from me... Not a lot, but we can keep the discussion going at least and see what we can come up with.