Is 15 minutes enough time to change trains in milan to venice?
Yes, if the first train is on time - and there aren't any slow walkers/tons of luggage in your group...My last fast train into Milan (all the way from Florence) was 2 hours late...grrrrrr...We got to Paris the NEXT day. My advice to you would be to go find the conductor/ticket checker if you see you're going to have a problem, and get his/her help while still on the train. If necessary, you can book two separate tickets to increase the time between travel legs.
We had the same problem. So when you get off the train there are about 20 or so rail platforms. There is a HUGH timetable board in the station right in front of the platforms. Check the board for your next train to tel you which platform to go to. It is really easy. People move fast anyway but if they don't do what the Europeans do and use you're elbows or push ahead. There seems to be no manners but go with the flow.
Be at the door when the train pulls into Milan so you can be among the first people to get off.