It depends on the interviewer. I think it’s fine to email him with additional questions. You should be respectful of his time but use cues from him as to how much time he wants to continue to invest in talking to you. I would stick with email, not text, unless he texts you first.
Our S19 interviewed with a young alum at Carleton and, even though he didn’t end up going to school there, they still keep in touch. The alum called S19 after he was accepted and asked if he had any more questions. S19 was honest and listed his options and they talked through pros and cons of each. They even met again in person. I think they are keeping in touch. Won’t hurt to have him as a future connection.