They are considered relatively peaceful. They will however eat newly hatched fry of most species. Some other species of fish may nip at fancy guppies' fins. A few gallons per guppy is adequate. They generally live a few years.
Guppies tend to stick to the upper region of an aquarium.
They can survive, at least for a while, in almost any water. F. Be sure to provide lots of plants in which the females and fry can hide. Guppies like fine leafed plants like cabomba and a dark gravel bottom. They need at least a few gallons a piece. They eat most anything but prefer live insect larvae and fry.