Just thought ide drop a little bit of knowledge in on this.
A little while ago i ran basic analysis on the best sections to use for perm piers, and contrary to popular beleif a circular section isn't the best at all. Great in torsion but in bending not so.
If you can get your hands on 200 x 5 SHS which should be readily avalible from your local steel merchant, it will proform better. As for bolts and threads, i woudln't have gone anything less than M16 as M12 like has already been said will move a bit.
here is the link to my blog talking about it