If you really want to breed live bearers, such as your guppies. I highly recommend keeping Hornwort in your tank. This plant is very cheap and easliy avaiable at any petstore you go to. This plant will float nicely on the top level of the tank. The fry can easily hide in it, and it will help the adults from fighting as well.
Feeding live brine shrimp will help your fish breed very well, any live food will do this. But as well, you do want to do regular water changes. I do about 20% every week, and some times twice a week. I raise guppies and Endlers and they do seem to respond much better breeding wise to the water changes. But regular water changes is a good habbit to get into. And finally, I would keep them in a Species only tank. Just keep the one species of fish to this tank. Don't mix any thing else to it except for maybe either Amano Shrimp or snails for alage control. I would stay away from Ghost Shrimp.. stay with Amano.
1. So feed your fish a varity of foods, dry, frozen and live.
2. regular water changes.
3. Live plants and plenty of coverage.
4. a species only tank.