In Age of Empires, almost every country has a musketeer, a hussar, skirmisher, etc.
In Revolutions, it's slightly different. Take the Russians for example. They get a infantry unit armed with a musket (historically accurate uniforms, of course). While he's a lot cheaper than other nations' infantry, he has a lot lower ranged attack (the Russian Army didn't do much firearm training; it was said veterans of 20 years had never fired their muskets). Two of the formations that are available to this solider are the attack column and company column. The company column is similar to a line formation, but it's not as effective for the Russians. The attack column is like a mob formation, for the purpose of charging the enemy. These were real formations employed by the Russian Army.
Of course, the American army isn't going to use the same tactics as the Russian Army. Not only does the American Regular look different, it uses different tactics and weaponry. While American units are available in large numbers (only in later ages), the Russians still have more. However, the Americans have more effective industry and technology. Even a regiment of militia, short on health/morale, could do considerable damage because they can outshoot the Russian infantry with rifles.
EDIT:
I forgot to mention where upkeep comes into play. Even the cheapest American unit requires more upkeep than the standard Russian infantryman.
Mexico has a very low taxation rate, but most of it's units only require food as an upkeep. Britain and France, however, are pretty expensive.
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[This message has been edited by Captain_Soanso (edited 07-10-2007 @ 09:19 PM).]