Depending on how many you want to get will determine the tank size, so how many are you wanting and are you wanting to have fry.
Guppies are livebearer and the female usually drop fry about every 28-32 days more or less, both the males and females can and will eat the fry as they drop from the female, some do and some don't-have good cover for the fry will help more survive.
I like to use at the least-10g tank for a trio-1 male and 2 females and soon this number will be more as the females drop 20-80 fry every month
Guppys like hard water but usually will do fine and adapt to most hardness levels, like most fish this can vary.
Water temp-I keep mine in 70-80F water-depending on what tank they are in and time of the year
Guppies are a pretty hardy fish and eat just about anything
Some report that the fancy tail guppies and Bettas don't do well together due to the Betta confusing the fancy guppy tail as a Betta, however, I have never had any problems with my Bettas confusing guppy fancy tails as another betta, but anything can happen.