Question: Near the end of the song "I am a dentist", is Orin saying "and a success" or "and I say sit"?
Question: When Seymore is having the final confrontation with the plant, the plant sings, "You can keep the Thing. You can keep the It. You can keep the Creature, they don't mean shit!" I get that "the Thing" is a reference to the monster from "The Thing From Another World" and "the Creature" is a reference to Gillman from "The Creature From The Black Lagoon", but what is "the It" a reference to?
Answer: Most likely it is referring to It! The Terror from Beyond Space (1958) or possibly It Came from Outer Space (1953).
Answer: I'm assuming that "it" is simply a vague reference to non-human creatures. (It most likely wouldn't be a reference to "It" by Stephen King, as "It" came out after the song was written.)
Answer: He was speaking in general. It being used as a subject, direct object, or indirect object of a verb, or object of a preposition, usually in reference to a lifeless thing.
Answer: Seems like "It" is the titular creature from Stephen King's novel "It." It was a shapeshifter, but mainly took the form of Pennywise the Clown. The novel was released 3 months prior to this film's release.
Answer: He says "and a success". He's singing about his mother telling him he'd become a successful dentist.