One of my ideas was to lead the player through some of my ideas for games/games I already released. 

Would it be ok to use some of my older assets from the games to be in game sets? (Kinda like a stage.)

The assets would not be used for anything else. They would only be found in their respective areas.

Also, the code for the door in Chapter 3 is 1587, so if your here from the game in the future, have fun!

Yup that's fine, just mention what you reused when you submit (there'll be a textbook)!