Tha best souce for dimmensions and materials of seatposts, would be the manufacturers themselves. Other than the diameter of tubing of a seat post or the material, they are very much alike. You have your suspension posts, then there is posts that have the seat reail mounting clemps directrly above the center of the post, then there are some (mojority) that have the mount slightly back of the center of the seatpost.
Neverless, you should just use the different tipe of seat post to properly set up your bike. Front of your knee cap directly over the center of the pedal spindle. Also, you want full leg extension on the downstroke; with your heal on the pedal you should be able to fully extend your leg. With your bike set up you will strain your knees the least, and also get the most power out of you pedal strokes.
Yes, it will feel too high at first, and no you will not need to lower your seet for those nasty downhils. If you have any other question regarding set-up, make another post. [sig][/sig]