It can vary from file to file, but these would be made off of the same 3d model, so as long as the scale (at the bottom) is the same (which it is), they should fit/join/etc. together properly.
EDIT:
I daresay by the looks of it, the chest brace is to go underneath, so the main chest has something to sit on (could get a little awkard trying to keep it all on without a heap of padding, maybe?), could also be to give you something to attach a miniaturized arc reactor to?
The files are probably all from the same suit and/or modeller, and if not it's your suit you can have it look however you like.
As long as the pieces are scaled correctly, they should fit together fine.