Assuming you're playing for the maximum number of top brass (i.e. 5 heroes), then in your initial team you will need a pig that can detect mines by 'The Village People' at latest - so you can find a safe route that doesn't lose you a pig or three and get the special medals in minefields.
So promote 1 of your pigs through the Engineer career, or get a commando by then (creating a VERY unbalanced squad). Also, you must ignore the 3 spare pigs if you are to achieve having 5 heroes (Therefore, your main squad cannot change throughout the game).
Finally, you will also need all 24 survival bonuses and every special bonus in the game (excluding the dodgy one on Geneva Convention).