If you are trying to separate the fry of various females, it does make sense to have a female drop by herself. Have you gone through the Immediate Help links under breeding? There are a lot of suggestions there as to how to save the fry without killing the female by moving her.

Out of curiosity, what sized space would you be moving her into? It doesn't sound like you would be moving her into one of those little plastic boxes one wit called "the little box 'o death," but too much confinement can be dangerous for the female.

What other fishes are with the pregnant female? If it is just that other female and both of them are well fed (again see the Immediate Help sections for specifics) and there is a lot of shelter in the form of plants or mops, many fry would survive right there. Even in the wild, where guppies may be hungrier than in an aquarium, fry survive. :)

How many fry do you want? If you have 20 fry, you need at least a 5.5 gallon/20.8 liter tank to get them started. In time, a 10 gallon/37.9 liter tank would be useful to "grow out" that bunch. If you are separating males and females, so you can select your next breeders, two "10s" is not a bad thing. Space and tank needs can quickly get out of hand however. :)

Your questions are terrifically important for you and for your care of your guppies. They are close to what all of us have had to investigate and answer, with a little coaching, for ourselves.

Take a look at the small but very insightful introductory commends by Stan Shubel at his site.

http://guppiesbystanshubel.netfirms.com/Intro.htm

He suggests that most of the "secrets" of guppy raising are merely common sense. I'm sure what is common sense to experienced guppy people is not immediately common sense to many of us getting into the hobby. Then later on, several mental thunderclaps and "duhs!" will take place and some of these things will fall into place. :)