you can have them intersect/go through one another with no real issue. i usually perform a quick cut operation though so that the one object doesn't go through, but appears to go through. (oh, and face count is the standard for counting stuff in games modeling...worry about that). its the illusion of being attached, without being attached.
and also, trust me when i say this....any shape with the right amount of thought and time can be achieved with low poly....i should post up some of my original designs. in fact, yeah..i'll do that here in a bit.
edit: here's the hull section to one of the my original designs. when you're trying to model something, prioritize what needs to looka certain way, where the emphasis of detail goes. usually, if i'm modeling a complex area, i set aside at a minimum 1500 (3000 tris) aside for it and whittle it down. i go through crap loads of revisions until its as efficient as humanly possible without degrading the overall. also, don't be afraid to use a little sub-patch (or mesh smooth for you max heads, especially you max folk....max's meshsmooth is freaking god like) to acheive a desired look. on a particularly advanced organic shape, use it. use it sparinly. i actually used lw's sub-patches to build the bulk of my strakha, and also to form the hull of my Salthi. i cleaned the excess and then continued on my merry way building em. i need to do up a sub-patch tut one of these days i think...its insanely poweful if used correctly. oh, and tri-count for that hull piece is only 1766.