The game selects the soldiers at the 'top' first. Once those become wounded / unable to fight / dead the list goes further down. When the battlefield allows for more of your party to enter, the soldiers at the bottom are moved in (from what it looks like now).
I would suggest:
1) either having a percentage of specific unit, which players can set (eg. 40% unit A which is cavalry, 40% unit B which is infantry, 20% unit C which is ranged units)
2) allow for separating the same unit group into two. (eg. currently if you have 10cavalry of the same 'tier' they would be stuck in the same group. Separating those 10 into 4 / 6 in different groups would allow for something like this:
- group 1 - Player
- group 2 - 5 unit A
- group 3 - 3 unit B
- group 4 - 4 unit A
- group 5 - 5 unit C
- group 6 - 7 unit B
etc. This would mean that if the battlefield supports 10 troops, you would end up with player, 5unit A + 3unit B + 1 more unit A on the field, hence would allow you to select relatively how many of who enters the fight..