The word is going to be out soon, what with the four DC CW casts for Arrow, Supergirl, DC Legends Of Tomorrow and Flash all filming together right now….
It has been established that Arrow, DC Legends Of Tomorrow and Flash all take place in the same fictional universe, but Supergirl is from another dimension who has to cross over.
As of the upcoming Flashpoint crossover storyline, Supergirl will now be a part of the main DC TV universe. Only Barry Allen will know that they were once separate… any remaining continuity issues will be handwaved as "a time traveller did it".
Also in Supergirl, We know Metallo is the bad guy but they are introducing a new Metallo 2 character. Both Superman and J'onn J'onnz will fight him, while Supergirl deals with the original Metallo on her own, elsewhere.
Look out for Krypton Park in Metropolis…