Hello L...
You don't need a separate tank for your fry. I keep large, planted tanks of Fancy Guppies and keep all the fish together. When I see fry in the tank, I just feed all the Guppies a little more and a little more often. I've found that well fed adult Guppies show little or no interest in the fry. Plus, the fry grow faster with several, small feedings a day.
I also float a lot of Anacharis and Pennywort in the tanks. It makes good hiding places for the pregnant females and the fry. The more floating plants the better.
B