Yes but they can and do go to the bottom of a tank too.
You do not say if the guppies are common guppies or fancy, also how many male and female guppies you have, yes as far as the snails and guppies go, one point to remember is the tank water should be above 72 degrees and some live floating plants for the guppies too hide in.
As soon as they are too large to be eaten by the main tanks inhabitants.
No, this is blatantly false - very few fish will 'grow to the size of the tank'. If the tank is vastly too small they will stunt and live lives of deformed misery. The minimum tank size for guppies is 5 gallons (which would allow you to have four.)
It is not TOO cold, it is normal.
Guppies are great community fish and can live quite happily with most Tetras, Rasboras, Barbs, Anabantids, Dwarf Cichlids, Danios, Corydoras and many other species provided they don't get too big and eat the guppies.
If the water temperature does not get too cold, and the water continues to be aerated, then the darkness won't matter. Guppies survive perfectly well in the dark.
Depending on the degree of hyposalinity, or your definition of "too much salt", it could kill the tank inhabitants.
no
Guppies mature differently depending mostly on tank water parameters. For example a colder tank will slow the maturation of your guppies but will probably also extend their lifespan. When your males start to colour up-they are most likely ready to reproduce, this should be in the 2nd or 3rd month. Females will be ready at this point too. Your males will be approx. 2 centimetres long, females a little bigger.
The girl guppies are going to prefer the most brightly colored boy guppies. The plain color dull boy guppies will reproduce less. These two facts will continue as the guppies continue to reproduce. At the same time, those brightly colored boy guppies won't be happy with other brightly colored boy guppies, so there will be biting and bickering until some of the weakest brightly colored boy guppies will get injured and die off. The genes of non-related plain dull boy guppies won't get passed on, because they won't get a chance to breed; their gene lines will end as they die off. Assuming the tank and fish get enough filtration, air, and food, that tank will quickly fill up because guppies do like to breed. But over-crowding will kill off the weakest. Even the extremely plainest girls might not get picked by the brightest-colored boys so the girl population could suffer, too. And if more male guppies are born than female guppies, soon you'll have a tank of really grouchy guys who all want to be God-Guppy of the tank. Any girls left might even go into hiding to get away from the fighting. Eventually, there won't be reproduction, because either girls have died off too or the girls try to stay away from aggressive boys, and the most aggressive boys will kill off the weakest guys. Plus, guppies do eat their tiniest babies. So you'd end up with a tank of grumpy guppy males too scared of the God-guppy to even swim much..... making a very unhappy guppy life in a tank...
Yes, your six pregnate guppys give birth in your five gallon tank