A bit hard to judge when she´s due. Which goes for a lot of mollies that it´s not always that easy to estimate. A gravid spot on a black molly is not to be seen. You really have to take the bodyshape as a reference to make an estimation. There are a number of molly and limia species (both are related) that show a gravid spot and a number of species that won't show a gravid spot.
Well, if they square off, it will be a sign that she's really close. But to be honest, there are anough mollies that won't square off but stay round and will drop fry. Most of the times (so, not always) when it's the first batch, the number of fry will be low. Same goes, if such a female will change enviroment, she'll have a small batch "most" of the time.
Female mollies can drop fry after a pregnanct between 3 - 8 weeks. It depends on the kind of molly we're talking about. But most have a gestation period between 4 - 6 weeks.
While the development starting from fertilization till a fully developed embryo takes only a couple of days, means that the rest of the gestation period is meant for growing. That's why a female can drop fry earlier than 4 weeks. The newborns will be smaller in such a case.
It's also hard to tell if a female will drop a lot or just a few fry. Same goes for large or small fry if a female's belly is huge. A huge belly could mean a lot of small fry or a less number of fry but large sized. It can happen either way...
In the wild it depends on what kind of livebearer it concerns wether the females live in numbers. It wouldn't be correct to claim that all female livebearers would shoal in the wild. It really depends on the species. The enviroment in free nature where livebearers live differ a lot. From slow till still waters and moderate till fast flowing waters. So, the habitats can not be equally compared to eachother. One of things you can consider is the way they're build to tell something about the water flow or current they live in. Also the fact wether they're superfetative or not will tell something about the water flow they inhabit. There a serious number of species of livebearers that prefer turbid water above clean water. You really need to know the livebearer species in order to know what their needs are. There livebearers that prefer soft till neutral water and those which prefer neutral till hard water. This already means that not all livebearers need hard water. And not all male livebearers harass females all the time. Which means that there are a serious number of livebearers that can be kept at aratio 1m:1f or even more males than females. It's a typical generalization to say that livebearers need to to be kept at a ratio of more females than males at all times. It really depends on the kind of livebearers. And even within the same family, the behavior of subspecies can be different from species to species within the same family.
There's a whole world behind the discipline of livebearers...
