The episode title "All happy families are alike" refers to Leo Tolstoy's tragic novel: Anna Karenina.
The complete quote is: "All happy families are alike; each unhappy family is unhappy in its own way."
When Gordon, Bullock and Falcone enter the elevator leading to Barbara's apartment, there is a woman in the elevator with them who is holding a dog. This woman is played by actor Peter Scolari who also plays Commissioner Loeb. Scolari's earliest starring role was in the 1980's sitcom "Bosom Buddies (1980)". Scolari played Henry, who, along with his best friend Kip (played by Tom Hanks) must dress in drag in order to live in an affordable, women only, apartment building. It is unclear if this is an inside joke based on Scolari's previous role, or if he is indeed playing Commissioner Loeb, who may be in disguise due to the erupting Mob war going on in Gotham.
The classical music playing at the end of episode 22, Prokofiev's 'Dance Of The Knights', is very similar to Harvey 'Two-Face' Dent's theme in Batman Forever (1995) titled 'Two-Face Three Step' by Elliot Goldenthal.
The red and black chainmail piece worn by Selina Kyle was made by Poly's Pleasures Chainmail.
John Clarence Stewart is another DC-Marvel crossover actor. After playing Officer Griffin Katz here he became Alex Wesley in both seasons of Luke Cage on Netflix.