Welcome to the Forum!
IME, Guppies should not be in anything less than a 10 gallon (20" long). They are active fish and the longer the footprint the better.
As far as water changes, it helps to know if the tank will have a filter. And do you know about cycling a tank? Here is this Forum's tutorial:
https://www.bettafish.com/30-betta-fish-care/507585-cycling-two-sentence-tutorial.html
But a weekly 50% water change until the tank is cycled would be fine. SeaChem Prime is the key to safely cycling an aquarium.
IME, Guppies should not be in anything less than a 10 gallon (20" long). They are active fish and the longer the footprint the better.
As far as water changes, it helps to know if the tank will have a filter. And do you know about cycling a tank? Here is this Forum's tutorial:
https://www.bettafish.com/30-betta-fish-care/507585-cycling-two-sentence-tutorial.html
But a weekly 50% water change until the tank is cycled would be fine. SeaChem Prime is the key to safely cycling an aquarium.