You may want to use up any flake food package within a month or two. Any vitamins, such as vitamin C, are destroyed by light and humidity "they say" by three months. A more vitamin-enriched diet again enables the fish to put the food to better use. If you can feed a special treat of blackworms or rinsed, formerly frozen brine shrimp or bloodworms or glassworms, that will also diversify their diet, give them more protein and useful trace elements.

Even if you and I ate "the perfect food" all of the time (probably broccoli lasagna), we wouldn't necessarily benefit. And we'd be bored to death. :)

People breeding guppies for show, may feed (what they can eat and no more in a couple of minutes) even as often as 4-6 times a day. They also do partial water changes a couple times a week to even a daily pattern! Part of the increased water changes must be done to deal with the increased feedings. Part of it is that the cleaner the water, the healthier the fish. (Useful trace elements are more likely in the water.) They also spend less energy fighting off illnesses.

Like you, most of us are pleased to keep our aquarium water within safe perimeters. However with more effort the fish grow faster, are virtually disease free (if new acquisitions are quarantined a month and maybe treated for parasites). Colors may be stronger. Fry will be more numerous, a tad larger, healthier and more vigorous.

One of the special reasons for the increased feeding and care with swordtails is that at a certain point, males will develop swords. When they do that, most of the growth of the body stops. If they were stunted and slow growing when the swordtail comes in, they will be dwarves for the rest of their 1 to 3 years.

Raising temperatures is always a little controversial. The life expectancy of a guppy at 80 degrees F/ 2 C will many times be less than a year. The same fish, at 75-6 degrees F/ 24C might live 1.5 to even two years. But the increased temperature also increases appetite and growth, with feeding, will also be more rapid.

By the way, a small but very useful book is 1995's (and 1996's)  Platies and Swordtails : An Aquarist's Handbook
by Derek and Pat Lambert. Both authors (Son and Mum) have collected extensively in Mexico and use their experiences in the field and in the fishroom in that work. It pops up from time to time in the better shops. One could also go to the following and look around.

http://books.half.ebay.com/
http://www.fetchbook.info/
http://www.bookfinder.com/
http://www.seahorses.com/index.shtm
http://www.powells.com/
and perhaps the best deal from my quick survey:
http://www.fishbookstore.com/product_info.php?products_id=226

Derek, in his later 40s, passed away about a year and a half back, The livebearer hobby lost a real go-getter when that happened.

First compare your fry with newborns at the shop or a friend's place. If they aren't a little bigger (and at least the females showing their gravid spot) then consider your feeding and environmental perimeters.