Putting together the info we now have, you need to remove the guppies, platies and rainbowfish as your water is much too soft for these to be healthy. The other mentioned species will all thrive in your water.
It should help to explain about the shoaling species numbers, so I'll try to do so to help you.
All freshwater fish species have evolved over thousands of years to function at their optimum in a very specific environment. "Environment" here means the water parameters GH, pH, temperature primarily, also KH), the type of habitat (substrate, wood, rock, gravel, plants, light, water current), and numbers of their own. All of these needs are programmed into the species DNA, which means we must understand them and do our best to provide them if we want the fish to be at their best, healthy and "happy" so to speak.
Some species are shoaling, some are not. Shoaling species live in groups usually of hundreds if not even thousands. They "expect" this, and will be stressed if they do not have others of their own species. Numbers are not always easy to give, and the hobby tends to focus on minimum numbers, and with good reason. Many of us have recognized for years that a group of the shoaling species was needed for the welfare of the fish, but only recently have scientific studies been carried out, and not surprisingly they have confirmed what we suspected. If there are too few of the species in the aquarium, the fish are under severe stress. Most often this develops into aggressive behaviour; normally aggressive species become much more so, and normally peaceful species become aggressive. Sometimes the opposite occurs, when the fish just wastes away out of stress and fear. The fish really has no other way to deal with frustration besides aggression or withdrawal.
Reasons they need the group are linked to security--safety in numbers. Some species develop hierarchical interactions that are essential for survival (loaches are a good example). Some just "relax" by interacting, challenging one another, "play" if you like. Some need a group to initiate spawning. In every case, you will always be certain of observing much more natural behaviours from the fish of a shoaling species with more of them, and that is worth it.
Numbers can vary according to specific interactions of some species, but it is almost always going to be better the more of the species there are in the tank. Six is an often cited minimum, and if space is this restrictive, it can work. But the fish will be better with a few more, when the tank allows for this. Some species, like the rummy nose tetras mentioned, must have more; I would never have fewer than nine or ten of this species, and I tend to go with around 20. Corydoras catfish need at least five, but again, they will be much healthier with a few more.
Turning to your aquarium, which I will assume is 36 inches (90 cm) in length as it is 180 liters (176 liters in post #5). With the removal of the livebearers and rainbowfish, you can increase the numbers of the tetras and cories with no issues resulting. The fish absolutely will be healthier, and reward you with interactive behaviours.
