There's a high degree of randomness per game, but it's clearly not totally random (i.e., each map area has it's own restrictions as to troop type and difficulty that you will face).
As to troop size changing once placed, I don't think so. Note that the displayed difficulty (weak, impossible, etc.) isn't just a factor of your level. It's a factor of what your army is composed of, as well. It's likely that you picked up additional troops or switched to a powerful troop type.
As to mixed armies in an area, the game has a bit of a "figure out the maze element." It's clearly not designed for you to "sweep" an area all at once. Instead, you should pick off what you can in the north, then get what you can in south, then go back north.
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