Brushing teeth while fasting will not break the fast. So, you can brush your teeth using toothpaste while fasting so long as you are careful to avoid swallowing any of it.

There is nothing wrong with congratulating one another at the beginning of the month of Ramadan. There is nothing wrong with saying Ramadan Mubarak.

Popping pimples that appear on the face does not break your fast and you do not have to make up that day later on.

There is nothing wrong with a fasting person if s/he gargles their mouth with water, or with water and salt, especially if there is a need to do that.

There is nothing wrong with tasting food while fasting and it does not affect the fast so long as none of the food enters the throat of the fasting person. This applies equally to coffee and other things.