you can just put the GTR centre, halfshafts and hubs into your 33 without changing the subframe or the uprights.. it all bolts straight in. easy as.
and yes.. you can mix and match. im about to put GTR LSD from my 4.1 r32 gtr diff into a 4.3 r200 diff and run the GTR halfshafts. as its easier tyo change ratio by changing housing rather then CW&P
as for S13, R32 etc diffs. they will bolt into an R33, but you need to use the bolts that bolt it in... not the r33 ones. and you need to change the rear hat.
you'd be suprised what can fit in what.