Once they are pregnant (now), you can move them to a birthing tank anytime. This will let them relax and you can pamper them, resulting in larger spawns and usually better viability. I would use a minimum of a 5 gallon tank, and plan to remove the female the day after she starts dropping young. Any losses to cannabalism will be more than made up for by the increased size of the spawn.
With good conditioning, the female will get HUGE. You think she looks pregnant now, just wait! Plus, once she does drop, mark your calander. Guppies should drop fry every 25 - 35 days, so after 3 or 4 spawns, it will be easy to have tanks ready at the right time. Don't worry if you can't save every spawn. Guppies live for several years, and older females produce larger broods anyways.
I do this with my 'guppy types', and get 20 - 50 fry each month from each female. I keep the fry in the smaller tanks until they are 1 - 2 months old, and then distrbute them or return them to the main tank. Only have 2 tanks? You need more!
Alternatively, lots of folks just keep a colony of guppies in a 25 - 50 gallon tank and let them increase in numbers naturally. This works, with the key being a large tank with lots of plants, live or plastic. I have done this successfully in a 15 gallon, but cannabalism was high and numbers increased quite slowly.
Also, use sponge filters rather than power filters to avoid fry being mulched by the impellers. ATI Hydrosponges are excellent and inexpensive.
Good luck,
/Kris
