It's safe. The chamber is longer to accommodate 3" shells but it will also work just as well with 2-1/2". I've been shooting the 2'1/2" in my 3" Mossberg for years with no problems.
Where have you been buying 2 1/2" shells? I'm pretty sure your thinking of 2 3/4".
No choke . Fun to shoot both barrels at the same time. I love mine.