Can you have green tree frogs and guppies together?

You should NEVER mix Amphibians with fish. Fish need their water clean because that is the medium they breath. Amphibians poo and piddle in the water making it poisonous for fish. The rules for keeping fish successfully are :- 1 inch of fish needs a minimum of 1 gallon of water. :- Every tank needs a permanently running cycled filter. :- Every tank needs at least 50% of its water changed every week. Keep the above rules and your fish stand a chance. Fail in any of them and I can guarantee that you will have constant health problems with your fish.