That is very gracious of your Mom, offering to help get a larger aquarium for many of the fry. Somewhere along the way, you two will have to consider how many fry you can keep. As they get larger, more than one adult live bearer per gallon is getting pretty crowded. (and we have a Kelly-green tank with two or three Endler's per gallon.) You are certainly well aware of the problems that can come up there.

If a person really, really changes part of the water frequently, maybe a large part, they can keep remarkably crowded aquariums. I visited a friend who raised a lot of discus. I was amazed at the 90-95% he gave them, But the water was treated and the same each day. He used a garden (drinking quality) hose from the water reservoir. And because his filters didn't dry out and the water chemistry stayed the same, his discus, despite the stress of actually laying a bit on their side for a few moments, thrived.  

But a person needs to decide if the effort of these amazing water preparation and changes is worth it in terms of the fish they will raise and the energy and time they can offer.

This came up in a phone conversation with a friend of mine yesterday. He recalled an old article telling of an experiment by the well known discus breeder, Jack Watley. (If you look at a photo of him in TFH, you will notice that he is not a spring chicken. Another guy raises all the discus in his hatchery these days, while he gives advice, writes articles and speaks at events.)

Watley was to have put five small (quarter sized?) baby discus in a 20-gallon aquarium. I was told that he maintained that tank at 80+ degrees with regular feeding and more or less the usual daily large partial water change. (That may be usual for militant discus breeders anyway. I have heard of guppy breeders, preparing for the show season, doing close to the same.)

In a five-gallon aquarium he put a like number of youngsters. Feeding and water chemistries were the same. Supposedly he did SEVEN nearly complete water changes EACH DAY with seasoned, treated water. (This had either been pure research, a part of his job or he really needs a life away from the fish.)

At the end of a fair period of time (sorry I forget the precise month or two involved) he weighed the young discus, all of whom had grown. The ones in the smaller tank had gained 60% more in weight than those in the larger tank. In other cases they would have stunted and perhaps have been pretty unhealthy.

You raise an interesting question about the capitalization of fish names. Guppy is a common name (though historically there was a Mr. or actually a Reverend Guppy who was one of the early collectors of those fish.) Except for the usual conventions as in starting a sentence or titling an article, guppy the fish is lower case. Gambusia and Danio are the names of two genus. Usually they should be capitalized. But Danios to hobbyists though also include other small Cyprinids (or minnows) from southern and eastern Asia. In fact new species seem to be entering the hobby, especially from Myanmar (formerly Burma), a lot lately. They may be from several genus. (There's another "one of those things." The plural of genus is genus and then I begin making little b-b-b-b-b-b noises.)

I had the chance to amble through a couple of large pet shops today and both had very striking tanks of these genetically modified zebra Danios. Each aquarium of bright electric blues, reds and yellows was a terrific eye-catcher. Of course, each one of those transgenetic Danios cost the equivalent of four or five or six regular zebra Danios.

Gambusia affinis and Gambusia holbrooki (notice that the genus is capitalized and the species is not - that's just the way scientific nomenclature is written) are common to much of the eastern US. They have been mixed around and pretty indiscriminately planted in a lot of waters for mosquito control. While they will eat mosquitoes, they prefer fry, including their own. Unfortunately they have been responsible for local and state-wide extirpation (annihilation) of other smaller fishes. In a couple of cases they have helped other fishes (including other Gambusia species who were more attractive, but less competitive) become extinct. They are one of the factors which may lead to the extinction (or at least eradication) of the plains topminnow,  Fundulus sciadicus, in the next few years. They will certainly eat a lot of guppy fry and tatter the adult male guppies.

You might decide put the Gambusia female in the 1.5-gallon tanklet. I fear for her male in there though, if there is one. He may get torn up in a 1.5 so I don't know how many Gambusia (or Gambusias) could survive there.

I have a pair of adult Gambusia in a 20-gallon tank with the four darters I collected with them in S-E Missouri a year and a half ago. There are lots of plants but no top cover in there. There are also no Gambusia fry though I'm sure she has dropped several times. If the baby Gambusia stay high in the tank, their mother must get them. If they succeed in hiding in the plants, the darters must find them. In the wild they would get into really, really shallow and weed choked water and hope to avoid the predatory insects.

If I wanted to save Gambusia (probably affinis) fry, I would isolate her in a 10-gallon tank which was just stuffed with plants. She in turn would be fed with defrosted and rinsed frozen foods to the point where she literally couldn't eat any more. (One has to be careful not to foul the water.) I might then put a few live food organisms (blackworms, Daphnia) in that birthing tank. Then she would have little room for fry and if she did get a little hungry, would probably take the other live foods before her fry.

Still, you may need more Danios and other fish which can eat live bearer fry so that their numbers can be kept at a healthy level. It sound like you have plenty of those in with the female platy. In nature, if two fry survive to become parents, the original guppy "pair" succeeded in their task of keeping their species going. If a half dozen to a dozen of the potentially 100 to a 1,000 offspring survive (though a lot of creatures eat adult guppies too) they have been very successful indeed.