Sucking loach is one of those awkward names that can refer to several totally different fish. It can be a common or sailfin plec, a fish that grows very big and produces a lot of waste, so you would need about a 300 ltr tank. Or it could be a Chinese Algae Eater, a fish that grows fairly big and gets aggressive in old age, so not really a community fish at all.
Either way, the sucking loach needs a new home.
Mollies stay smaller but still grow too big for a 20 ltr tank. They also need totally different water conditions from tetras: tetras like it soft and acid, mollies need hard and alkaline and prefer a bit of salt (which is not good for tetras). Again, I would say rehome them.
Tetras are schooling fish and prefer to be in groups of 6 or more, but stay small. But they are sensitive to new tank conditions, so that is a problem. Don't buy any more now!
Guppies should be ok - but it is important to know what sex they are. Look underneath at the anal fin. Females have an ordinary fan-shaped fin, males have a rod-like thing. All males might be ok, though ideally they should be in greater numbers (but not in that small tank). All females makes for a more peaceful tank but they do grow larger so should really be in a bigger tank. A mixed sex tank should contain at least 2-3 females per male, otherwise the females will get too harrassed by the males- also a mixed tank means getting overrun with babies, and it needs to be bigger for the females to get a chance to get away from the males.
In short, buying "a fish" is like buying "a mammal"- you want to make sure you don't end up with a rhinoceros for that hamster cage
That's the first part. The second part is about providing suitable living conditions. If this tank has only just been set up and had such a heavy fish load added all at once, there is a big risk that the water conditions will crash and make it toxic to its inhabitants. Go to the top and read the pinned topic on cycling a tank. Basically, it takes a time for the tank to build up a colony of beneficial bacteria to deal with the waste of all your fish (and this is counting without the loach and mollies- a 5 gallon could never cope with all those). In the meantime you need to keep a careful eye on water stats. Get yourself a liquid test kit that tests for ammonia and nitrites (and nitrates) and test for those every day. Whenever either goes over 0.5 ppm, do a partial water change of 25% (if the readings are really high, do 40%). At first that will probably mean every day, but eventually you will be able to cut it down gradually to once a week, which is routine maintenance for an established tank.
The tank will only ever be able to house 3 neons and 4 male guppies- or better, a school of 6 neons.
