Chickens' bodies naturally secrete water resistant oils that the they rub around their feathers... so, chickens will not melt if they get a little wet.

As for 60F temperatures, that is pretty fine for adult chickens. My chickens are quite happy to go outside and forage in 50F, even when it is sprinkling. It gets cold where I live and my chickens don't start getting uncomfortable until it dips below freezing.
Chickens are very smart and usually know what they can handle... They will get out of the rain if they don't want to be in it... So, I would say to make sure that they have an easily accessible shelter available so they can choose to get out of the rain when they feel the need but other than that, they should be totally fine.
Note: Silkie chickens (breed) do not handle getting rained on very well because they are all downy feathers.