I just wonder how does stacking units differ from grouping them? Atleast in my opinion it doesn't sound like a good idea to allow even movement in stacks. What would happen when the "group" stops? Also its not realistic that infinite amounts of units can pass a narrow canyon between mountains in one turn.
When it comes to the length of games, you can always choose the quick game option. I don't usually choose it as I find it a bit boring to pass the early eras too fast. In my opinion there is nothing wrong with having longer games than in Civ IV as long as they're interesting. However, the modern eras aren't that interesting and I usually start a new game by then.