With guppies, if you get more males than females, you will need to get males only and then a fairly big group (at least 5) to spread aggression. Guppy males are very highly sexed, so a male would wear out a single female, particularly during pregnancy (and two males to a female doesn't really bear thinking of). This is why the recommended ratio is 2-3 females per male. Or an all male group (but no fewer than 5). Or, as I prefer it, an all female group.
Bettas (Siamese fighters) are not difficult as such, as long as you remember a few basic characteristics. The most important is that they are highly territorial. This means you can usually only keep one male per tank, and you can usually not keep a male with females. You also do not want to keep them with any fish they might mistake for another betta (such as a male guppy).
The second characteristic is that they have long flowing fins, so you don't want to keep them with any potential fin nippers (lemon tetras, serpas, tiger barbs etc).
The third is that they are slow movers and not very good swimmers, so a high current or very fast-moving companions can easily stress them out.
A male might actually be happiest in a smaller tank (but not a cup!) of his own, with a plant to rest in. Being slow movers and able to breathe air, they don't need quite as much swimming space as other fish. But you still need to keep water quality good, as with all fish.
You can keep several female bettas in a community tank, but you might want to watch them for aggression.