I've never played original PK, but I liked Field of Battle. I did a battle report for my first game of it, which you can find here:
link
The battle report doesn't go much into the rules, unfortunately. I read battle report online that did discuss the rules and convinced me to give them a try, but I can't find it now. Basing is pretty open. I use 5 stands for infantry, 4 for cavalry, because they can take that many "hits" basically before being removed completely.
The rules definitely aren't for competitive play, they are much more about creating an interesting battle, I think. As said, there are no point systems. Also, there are a lot of things that are not, or are only partially, in the player's control. Not only the card driven turn system, but other things. Like when a command group moves, they move from 1 to 3 move segments, depending on a roll and how good their commander is. So some will move farther than others. The results of fire involve each player rolling a die and comparing the two. If the defender rolls really awful, a lot of damage can be done. If the attacker rolls low, it will probably do nothing. So there is some huge variance there, and so you can't always count on a particular result. You really have to be prepared for anything.
I think it would be difficult to play this kind of game if you wanted to play pretty competitively, and have only your tactical decisions determine the outcome. But if you want a more story driven game, a lot of things will probably happen in the course of a game that would make for a good story. Particular officers frustrating their commanders by continuing to do nothing, when they could have acted and saved the day. Regiments making improbably stands against overwhealming odds. Or running away at the first sign of trouble. That sort of thing.
As it has already been said, there aren't a lot of particular special rules for different periods. There is one special thing for each period they cover. Mostly the differences for periods are part of the basic rules. So a particular period will mostly be defined by what units you are allowed to take, what formations they are allowed to be in, and what weapons they carry. But I think it would be easy to make house rules to further define things. For example, making mounted cavalry less effective in combat for later eras, when they were used more like mounted infantry.