The extended players, player 9-11, retain color when units are given to them, just like Player 12, they can have units given to them, their units can be killed via triggers, and removed, and just about anything else, they can even be ordered using triggers as long as you order them through another player's triggers, but you cannot create units for them, you must create it for another player then give it to them.
The reason why they aren't so useful is because they cannot have triggers run for them, so anything that happens to current player, such as AI scripts, cannot be used for them. You cannot set their death counts via triggers, but you can do it another way, you can create units for them in an area and have another unit kill them, and the death will automatically be added, it's not very convenient though, and since there is over 1600 usable death counts just for players 1-8, there really is no reason to HAVE to use them.
Players 9-11 start out as enemies to everyone, and visa versa, but they generally do not attack back, though they often will begin to follow any units that attacked them. The best theory as to why they cannot attack back is that they do not have vision to themselves, and it's not possible to give them vision at all, since there is no AI script for them to give or receive vision.
Player 12, unlike players 9-11, when preplaced is set to ally with everyone, and when units are given to Player 12, they also retain whatever alliances they had before they were given, along with color. Other than that, Player 12 is basically the same as players 9-11.
Any players past 12 can only be preplaced, sometimes they will crash when selected, like when their unit's portrait changes depending on player color. They cannot be affected by triggers.
None.