First thing, fisherton, is welcome to Tropical Fish Keeping forum, and the hobby.
Having read through this thread, I tend to agree with most of it, but will offer a couple observations. And the first is that the Betta really is not a community fish. I know others have had them with other fish, etc, etc, but the fact remains--as every reliable ichthyological source will tell you--that they are best on their own. Not in a bowl, but a tank with some live floating plants. A 2-5 gallon tank would do well, as Betta are not active fish.
On the corys, most will not do well at higher temperatures such as Betta should have. Some cory species can manage in warmer temperatures, such as Corydoras sterbai, Corydoras leucomelas, and a few others; but most prefer it cooler, around 77-78F seems to work although there are some that would be better even lower down the thermometer. And the dwarf species are some of these. I have my Corydoras habrosus at 76F which is as high as I would go. And as someone mentioned, a larger group with the smaller species is better. In a 10g you could do 9-12 of one of these.
I agree with not combining platy and Betta, though as I said nothing should be forced into a Betta environment anyway.
To the water parameters. Knowing the GH, KH and pH of your source water (tap presumably) is essential. The GH and KH will not likely shift much in an aquarium unless something is done to specifically target it. The pH will tend to lower, but the KH acts as a buffer. Selecting fish suited to your source water is always easier because it means stability and ease of weekly water changes. But, depending upon the numbers and the intended fish, some adjustment is possible. I won't get into that as it is a complex issue, but my article might give you some background:
A GH of 100 ppm is about 6 dGH, and this is soft water. If you were to go with livebearers, this should be raised as they need the "hard" minerals like calcium and magnesium. But stay with soft water fish, and plants, and you are in a perfect position.
The KH (alkalinity) at 40 ppm is about 2 dKH, so this is not going to significantly buffer the pH which should lower naturally, ideal for soft water fish.
Let us know the pH with the API liquid test; when testing tap water, out-gas the CO2 first. You can do this by letting a glass of water sit 24 hours, or by shaking some tap water very briskly for a couple minutes. Then test the pH. CO2 causes a lower pH because of the carbonic acid.
Hope this helps.