nik_n_dad1) how long of a straight should one have on either side of a plate girder or truss bridge before a curve?
It depends on what you have running into and out of the bridge, and what construction the bridge is. A through truss bridge may be too narrow for longer steamers with a lot of rear cab overhang if the curve starts too soon after the exit of the bridge. A little SW/NW switcher diesel won't be bothered, but a C&O 2-6-6-6 could have severe clearance problems. Ideally, the engine would be almost aligned with the track axis across the bridge when the front axle crosses over the bridge shoes. If you can't do that, try for half the length of your longest engine before the curve starts, somewhat less on a well eased curve.
I had a turnout very soon after a Pratt Truss bridge on my first layout. The points were maybe 2" from the leading truss members. One of my steamers scraped part of the bridge, so I had to thin the plastic a bit on the one truss member.
nik_n_dad2) how long of a straight should one have before entering a curve from a turnout (the main route, not the diverging route)
Through experimentation, you are likely to find this solution, and it applies to the previous question as well. A well eased curve could perhaps start as soon as the turnout ends...at the first join...but if you move directly to a fixed radius, you would want a minimum of something like 1/2 the length of your longest engine, maybe less. Mock it up and check to be certain.
nik_n_dad3) how long of a straight should one have in an s-curve (the length of the straight between the change in direction of the curve?
The general rule is the length of your longest item of rolling stock....but check for yourself. Body-mounted couplers on long passenger cars will almost certainly need one full car length, whereas shorter cars will need less, particularly if they have truck-mounted couplers.
Honestly, a good use of your time is in laying out some track temporarily and testing for yourself. Don't forget to check in the reverse direction, too....as in, backing the train through test zones. You might be surprised at what you learn.
-Crandell
Sounds like a pretty aggresive layout for a "newbie". If you are using the Kato unitrack, you should be fine with the sectional pieces. I would recommend a full section of straight around turnouts esp, as they are trouble spots for derailment. And you may want to lay it out with the track and test your longest cars.