Good questions. At this point your lack of unit selection is going to be your biggest handicap-- you need to be moving your avatar around on the campaign map and visiting new provinces so you can unluck new units, like naginatas, and warrior monk matchlocks. You are probably facing a lot of units that are "better" than what you have available, so that makes it tough. My win ratio's definitely suffered as I broke into higher funds matches.

Yari cav is of course superior to Light Cav man vs. man, but 2 light cav units gives you more versatility. Alot of your unit selection choices are going to come down to play style. If you are good at micromanaging then you might want more units to play around with. Some people like cav heavy armies, some like infantry.

Disengaging units to get another charge bonus is risky if the unit is in any way depleted or wavering, or if it is currently being engaged by the enemy unit it is trying to disengage from (obviously it is fighting the enemy unit, but what I mean is, if you try and disengage and the enemy unit follows you [i.e. if the enemy unit is actively targeting the unit you are trying to disengage] then you normally take a lot of casualties] . Sometimes its better to just let them slug it out. I'm not sure if you can disengage "slightly" and then re-charge-- meaning you wpuldn't have to wait for your unit to completely form up into a square again, but would just have to make sure that all its men are out of melee combat before re-charging.

I think you only get the charge bonus on first impact, and then only the actual men (sprites) in your unit that are situated so as to actually be charging. Guys in the third rank or who are still pathing wont get the charge bonus.

So I think if you are trying to break the morale of a unit, you might want to wait for your chargers to completely re-form into a square so that when they hit the enemy unit they will inflict a lot of casualties all at once. If you let them only "slightly" disengage then they are going to come back into melee combat piece-meal and although they will each (probably) get a charge bonus on re-establishing melee contact, its not going to all hit the enemy unit at once so the effect on morale would be less.

That being said, it seems that unit morale is too high and flanking penalties are too light in Shogun 2. So maybe it doesnt make as much difference as it should?

I don't know if anyone has done any scientific testing but I'll wager that AMP and Kocmoc have a very good idea of how and when their units are receiving the charge bonus.

I flank and send my cav in as soon as I can do so safely. If the enemy leaves his ranged units unattended by spears, you can bet I'm sending my cav in (unless I suspect a trap, as Cecil advises).