I have heard in the past that one praying alone may not say Ma'ariv/Arvit until shkia/sunset but that one praying with a minyan can start Ma'ariv as early as Plag HaMincha. Could someone provide me a reference for this? If one's custom is to pray with a minyan before shkia (ie complete both Minchah and Ma'ariv), can he daven both services together when davening alone? I thought I had read something in Kitzur Shulchan Aruch about this, though I could be wrong.
There are a few permits for davening early.
I would assume that the permit being dependant on a minyan means that you are in a community where the only minyan uses plag.