Until they die....
Depending upon their living conditions they can live around 4 years.
1.5 years
A female given good conditions can produce a new lot of fry every 6 weeks without much trying.
Guppies live up to 4 years old and when they die they do not float they sink down to the bottom of the tank
Yes, goldfish are known to get cancers on there stomachs and warts on there eyes. That's only when you have unclean or the wrong temperature of water. Guppies tend to live through these conditions better. However, if you have a tank with gravel, plants, filters, and fresh food/water, the guppy will live for a long time!
3 - 10 years in good water conditions
They can have a new brood every 6 to 8 weeks depending on conditions.
yes, they will live 2-3 years in tank like that.
Guppies are livebearers so they don't lay eggs.
20 years on average if in good conditions. The world record is 44 years.
Any fish may fight/bite if they are in cramped conditions and/or poorly fed. Guppies are not regarded as fighters but may sometimes do a bit of fin nipping of any fish with long flowing fins including other guppies and Bettas.
It depends on how many guppies, and how well you keep up the tank. In a ten gallon tank, I'd say no more than ten guppies, and do frequent water changes. 10-15% weekly. They need a filter and a heater, as well.