I couldn't find anything relating to Taylor Swift, but the following link refers to an interview with Idina Menzel regarding her role in Enchanted and why she didn't sing...
She said when asked:
"It's a compliment really that everybody misses my singing. Nancy was
never written with a song, honestly, so I think Kevin was a fan of
mine and honestly it was a compliment to be asked to just be hired on
my acting talents alone."
Sadly, I couldn't find any interviews from Kevin Lima himself...
...However, I did find an interview at AnimatedViews.com:
And this had mentions from Chris Chase, Enchanted's executive producer, speaking about Idina Menzel and her not being cast to sing in the movie:
AV: Indina Menzel is a very famous Broadway singer in shows such as
Wicked, but in Enchanted she doesn’t sing. Why’s that?
CC: We cast as an actress. That was one of the interesting things. A
lot of people have asked us. They know her so well as a singer and
they kind of wonder why would we cast her if her main purpose was not
to sing. Kevin and I are both fans of her from Rent and from Wicked,
and we’ve seen her in other things and we just thought she was a
wonderful actress and she felt like the perfect New Yorker. Just like
the contrast between Andalasia and, say, Times Square in New York, we
wanted a contrast between Giselle, who was a princess from Andalasia
and the woman who was quintessentially “New York”. So, it ultimately
felt to us that she was the perfect New Yorker.
AV: In an early version of the film, didn’t she have a song?
CC: Yes. We had discussed a song that might at the end of the movie,
between her and the Prince. You know what? With a different structure
of the movie, it would have been a great thing. But, ultimately, the
movie just couldn’t be longer. The movie couldn’t support it because
she’s not the main character. It’s really not a movie about her. I
would love to do another movie which is about her, because I’d love to
have her in a movie where she sings! But that’s the kind of hard
choice that Kevin always has to make. Sometimes, you have to let
things go that are wonderful things and you want to find a place for
them, but, you know, you have to stay true to the movie. The movie
tells what it needs.