Pallettf family: I wouldn't worry about not getting in with only your MVMCP tickets if you arrive around 4pm. The "unofficial" word from our experience is that they let people in to avoid a big crush of people arriving at 7pm.
When we went to MVMCP, there were fireworks at closing at 6pm, so most that were leaving were hanging around Main Street, which if you think about it - a pile of people going and a pile of people coming would make for major conjestion at the main gate.
When we went with our LTT PS for 5pm, we took our time walking down Main Street, arrived at LTT to find that they weren't ready for us, so we hung around Frontierland for awhile. No one even asked why we were there.
My advise: Don't stress it. Arrive around 4pm to give yourselves pleanty of time to get to your PS without rushing or worrying you will miss it. Not a fun way to start your MVMCP experience.
I'm sure they won't even question it.