A minimum of 30 gallons is needed, but adult size should be considered as some species will outgrow a 30 gallon aquarium.
Damsels can be aggressive towards the same species and often become more territorial as they age. Damselfish can be compatible with angelfish, blennies, gobies, hawkfish, puffers, tangs and some wrasse.
Green Chromis are one of the more peaceful species of damsels and can be kept in schools of odd numbers in the appropriate sized aquarium.