Your question has been merged with Everything Park Hopping. Post 1 has a lot of information re: hopping.
Yes, you must tap into the park where you have a reservation first, regardless of what time it is.
The one difference re: hopping at WDW compared to DL is the distance between the parks. Travelling between parks takes time. You may wish to choose only one park for the first day rather than hopping.
Our family likes to start our trip in the Magic Kingdom, especially if our first day is a partial day.