Sorry, FF, Flat rate is always the same - you can't change it, and it isn't being changed on you. It does vary on the combination - 3 reds gives 4 armies, 3 greens 6, 3 blues 8, and one of each colour gives you 10.
no, please refer to the how to play section at the top of the page
Bonus Cards
Escalating sets are worth 4,6,8,10,12,15,20... (i.e. the first set of cards turned in by any player is worth 4 armies, the second set is worth 6, etc.) Flat Rate sets are worth 4 for red, 6 for green, 8 for blue and 10 for mixed.