Thank you to for finding what you could before asking questions. There will always be a little redundancy and that's fine. But it is nice not to get asked which is the female and which is the male. ;)

You have thought through the crowding issue and the filtration and your frequent water changes will allow you to stretch the carrying ability of your two tanks. You friend is correct that we can stretch a tank's ability to support fish with almost daily 45-50% partial water changes. (I do this with wild-type livebearers and bright green bowls from which I take as much as 50%/day of the greenwater to feed to Daphnia.)

Indeed, if time, circumstances and interest even get you curious about breeding egg laying fishes (I'm thinking of especially killifish and rainbowfishes, but this also applies to many more) water changes and good water quality will enable you to breed a number of fishes, which are considered "problem fishes" in the books. Many fishes in the tropics don't breed in the dry season - a logical thing because hiding places and food are at a premium. They spawn after the rainy season gets started and the rivers overflow their banks into the flood plains. Essentially (and this can even have hormonal effects) the rainy season is one giant three month water change. :)

The flip side of that is that collectors will begin collecting when the flood waters begin to recede and the YOY (young of the year) have put on some size. In parts of South American that would correspond to our summer/fall or winter/spring for them. By December collecting is easier, though more risky. Fish like piranhas. despite their fearsome reputation will flee people when the waters are high. If they are cornered in a pool or oxbow lake, isolated from the river by drying, they will panic when people enter the pool with a seine net and someone may get seriously bitten, mostly by accident.

In addition to behavioral changes such as panicking and bullying, there is an increase in predation. Unless it is a really rare fish, I'm reluctant to even buy one, obviously wild collected in our late winter or spring, because it is far more likely to be infected with parasites or diseases. As the waters recede, water quality declines even in the wild. At the same time the water quality causes the fishes' immune systems to be compromised. The less effective immune systems can't fight off all of the organisms attacking the fish. In the North American Midwest, fish native to our area suddenly may exhibit an attack by the much feared columnaris because the heat and (with the declining photo cycle) dying algae are limiting the oxygen in the water. When tanks in a whole sale house or in a store aren't up to snuff, it is unfortunately too common to see outbreaks of almost uncurable columnaris in our new purchases. (That is a strong argument for quaranting new fish, so they don't take out everything else in the aquarium.)

So in a sense, conflict, vulnerability to predators and death by various illnesses are a direct result of shrinking habitats and declining water quality. If the population isn't adjusted any other way, epidemics will happen to adjust the population to the water supply. Unfortunately in isolated pools and in some crowded aquariums, the disease cycle happens so suddenly that everything perishes.

I would guess that you can carry about double the fry in the two-gallon tank, so maybe 4-8 can grow up beautiful and full sized with almost daily partial water changes. But fancy guppies do need clean water and some space to develop their finage. I can get away with crowding on wild Limia or Endler's livebearers (maybe Poecilia wingei or maybe just beautiful stains of wild guppies) which would never work with fancy guppies.

We don't like to encourage cannibalism, but an aquarium will reach a certain population density and then guppies which had previously left fry alone will begin eating them or will eat more than preciously. That is almost necessary or else all may die of some "creeping crud" which may have been in the water all along "at a non-lethal level."

Shops, when tank space, rent, pay to employees and utilities are factored in, can not justify raising fry except in the case of a really exceptional guppy or rare fish. Even with the great looking black mollies I swapped one time for a canister of food. I was dismayed to look back and see one being dropped in the ribbon eel tank as I was leaving the store. Baby guppies too, may just end up as feeders.

A pair of wild guppies might produce a couple hundred fry in their lives. If one to three pairs survive to replace them and have fry, that would be considered a success in terms of maintaining the species. We eliminate most of the predators in an aquarium. Then we are stuck with the dilemma of feeding and housing all the fry, letting them get eaten, starving them, practicing euthanasia on the less desirable ones or watching them perish of disease.