Can you wear flats (shoes) with leggings?
Yes, in the spring, going for a casual look, the leggings will look great with a good flat shoe.
Can you wear flats with dresses?
Absolutely. A lot of women avoid wearing high heels if possible. Just be sure to make the flats look good and match color wise
Are flats considered business casual?
They are not necessarily consider business casual unless you wear the rounded toes. Pointy toed flats are better for the office, because they have a much more polished look.