Made obvious in several scenes in this episode (and Virtuoso (2000), for that matter), Robert Picardo, the Doctor, is an accomplished singer. While he was at Yale University, he was a member of the Society of Orpheus and Bacchus, the second longest running undergraduate a Capella group in the United States. Then in 1977, he made his Broadway debut. He appeared in Gemini (1977) and Tribute (1980).
Dr. McCoy's famous phrase from Star Trek (1966), "I am a doctor, not a _____" is referenced when the holographic version of Dr. Zimmerman states, "I am a diagnostic tool, not an engineer!"
The Doctor is signing the duet "O soave fanciulla (O Loveliest of Maidens)" from Giacomo Puccini's opera "La bohème", the translation for the opening lines sung in the opening scene are: Rodolfo (Doc's character): "O loveliest of maidens, O sweetest vision, Bathed in the soft glow of a moonbeam; In you, I see a dream come to life--A dream I pray always to dream." Mimi: "In the depth of my soul I tremble with the height of passion. Your kisses thrill Love itself!"
The swarm ships are described as miniature by Janeway and others, but as revealed in The 37s, Voyager's saucer section is arena sized. Its trunk and nacelles are the size of large buildings, which would mean that these "miniature" ships are covering large areas of Voyager, making them rather large vessels.