If you have unusually thick walls(i.e. plaster -deeper than 3/4") you may need one of the better stud finders.......they will say "Deep scan", which gives you twice the scanning depth of standard ones.
some will even allow you to scan for piping/electrical runs. Expect to pay more for these units.
I think 200's would work fine for the surrounds. I use a similar sized, sealed speaker for my surrounds. My mains are 170's, and my center is an older B&W piece.
Ed
* Sierra-2EX's W/V2 crossover upgrade
* (2) Rythmik F12's
* Parasound Halo P6
* Audio by Van Alstine DVA-M225 Monoblock Amps
* MiniDSP 2x4HD For Sub calibration
*World's Best Cables Canare 4S11 speaker cables