You are right to raise the issue of length, too many women are riding bikes which are too long.

You say you need a WSD (women's) bike. How big is the frame?
If you are a newbie, how do you know that you need a 50cm TT?

For a small bike, 1.7cm is quite a long excess in TT, you have to think percentages rather than just absolute distances. If you are chosing a new frame, then pick one with the right TT length. TT is THE most critical dimension on the frame.

Although you "size" a bike by its seat tube, as long as you have standover clearance, you can adjust the saddle height. The modern style is for smaller frames with more standover, and for compact/sloping TT designs. Generally if you are inbetween 2 sizes, you should select the smaller size.

Dont adjust the length of the bike using the saddle, you need to set the saddle relative to the pedals, THEN set the bars relative to the saddle. If your cranks are excessively long, then this will affect your saddle position. You have to start the sizing process with correct size cranks and correct size top tube.

You have +-2cm of adjustment in stem length. Shorter stems will affect the steering, making it more sensitive to input, but the only way to see is to try it. Make sure you use a stem with a front-loading design and pick a bike shop that will let you swap stems around to try them whilst riding.

Most guides to fitting are more relavant to athletic males of average size who want to race. Have a read of some more comprehensive and well thought out guides at:
www.peterwhitecycles.com/fitting.htm
and
http://www.myra-simon.com/bike/tips.html