well, stacks r ordered by the power of the units on it, if ur sprites has 500 ks of battle power & ur mr 300ks the sprites will be ur first stack in the battle, if u summon 300 ks of battle power of mr the mr will be ur first stack and the sprites the second,and remember that in battle ur flyings units increase his power * 2.5 , melee units * 1.5 and ranged units * 1, so ur sprites has more battle power thant net power, u can see the order of ur stack in battle looking ur status report and apply the battle bonus on the % of power of ur units.
P.S: sorry for my foul english.