Levanto to Milan then 15 minutes to change trains to Venice... Is that enough time??!!

Yes it is. If you are traveling on a single ticket, you should be protected to get on the next train to Venice if the IC train is late arriving in Milan. There are fast trains to Venice every hour.

The layout of Milano Central station is pretty straightforward. It's one big room, so you walk up to the head of the tracks and down the row to the next platform. To give yourself full use of your 15 minutes, try to be waiting by the door of the train with your luggage before the train stops. You will see others doing the same, so don't be at the end of the line.

Since most of us are trained by the US airlines, we worry. But actually 15 mins is a long time for a train change. Know the number of your next train so that it will be easy to spot on the departure board.