With guppies, if you want to be able to give away / sell your fry to the LFS or other hobbyists, it's best to try to keep the offspring to the same strain. When you buy the females, try to buy females from a tank that either contains only one strain of females and males or from a tank of females only, where none of the females are showing signs of pregnancy and you can then purchase your own males of the same strain. What you want to avoid is taking home a female that gives birth to lots of mottley guppies that you can't then sell / that the LFS wont accept.
The breeding tank for the female doesn't need to be big (18 inch tank is fine), but the growing on tanks need to be larger. Really you need two 3 foot tanks for the fry, if the growth isn't going to be stunted, although you can make do with 2 foot tanks at a push. I say two tanks, because you really need to separate the fry into single sex tanks as soon as you can to prevent further in-breeding, normally at about 6 weeks.
You'll need to carefully watch water quality and avoid cycling the tanks with the fry if you want to end up with healthy juveniles.
The other thing I'd suggest is not using a breeding trap. Most commercial breeding traps are really too small for a female guppy and will unduly stress her. Either make a larger version of your own out of a small glass tank with a couple of pieces of glass and sealant, or just plant the breeding tank heavily so that most of the fry don't get eaten.
Be warned, with one female guppy you could easily be having 25 fry born every four weeks and once you start, you're committed to growing them until they're large enough to sell / pass on. If you have two female guppies, that's in excess of 50 fry per month.
Having successive generations of fry growing at the same time, can lead to even more tanks......
If this sounds like far more work than you want (I tried it for a while and it was!) then just leave the females in the community tank. Plant the tank so some of the fry will be able to hide and accept that most of the fry will end up as food for the community fish. You'll probably have a few grow on each month, but that's it.
One more thing, if you plan to give / sell the fish to the LFS, check with them first that they'll take them. You don't want to be left with 150 guppies and nobody to take them. I used to exchange mine for credit at the LFS for other supplies.
