Is it possible to exercise Metta to all or is Metta only a mental state only? If so how does one act to remain dispassionate?
"Practise yourself, for heavens sake in little things, thence proceed to greater". Epictetus (Roman Stoic Philosopher)

You have to use wisdom in order to make metta stronger and you have to practise metta formaly and informaly.

With wisdom I mean perceptions you have to reflect upon many many many times:

• No one can escape aging, sickness, death as well as mental and physical suffering

• Everyone is heir of his/her thoughts, actions and emotions. If one acts badly it's because of the causes and conditions that are meeting then and there