Also, was there a feeling of breaking a barrier when he was elected? I can't imagine how my 85 year old Italian immigrant grandmother would react if a Catholic was voted in.
In short, yes, very much so. It was one of the main sources of ammunition for his opponents regardless of position. He was repeatedly asked about it going all the way back to his Senate days. Here's a quote from Kennedy during his presidential run in 1960:
"I am not the Catholic candidate for president. I am the Democratic Party candidate for president who also happens to be a Catholic. I do not speak for my Church on public matters – and the Church does not speak for me."
Kennedy especially needed the strength of LBJ in the South due to his Catholicism (and some other matters, but you explicitly asked about his religion).
Another quote during his presidential run:
"No one asked me my religion [serving the Navy] in the South Pacific."
When Kennedy lost the Democratic nomination in 1956 his father thought:
it just as well that [John F.] Kennedy lost, due to the potential political debility of his Catholicism and the strength of the Eisenhower ticket.
Here are some sources as well:
Kennedy, John F. (June 18, 2002). "Address to the Greater Houston Ministerial Association". American Rhetoric.
Reeves, Richard (1993). President Kennedy: Profile of Power. New York: Simon & Schuster.