For extra clarity (and because I still can't edit my own messages):Question summary:
* 4mm (8/23") nuts&bolts are fine instead of 3mm ones (6/32") and will in fact improve case/fan/hard drive stability?
* are washers (next to bolt and/or nuts) a good idea?
* I've ordered rubber gaskets for between case/fan (and hard drives are attached to the fans). Is it a good idea to place more rubber gaskets between the fans and the hard drive racks? - I'm mostly thinking about hard drive safety/life there, reducing vibrations to the hard drives from the fans
First off, thanks for the reply
So to summarize your post:
* bigger nuts/bolts are fine or better (not specified)
* washers are unnecessary and simply add cost/weight/...
* rubber does reduce sound but is unnecessary as far as hard drives go (though it can't hurt either)