Re: One general per city?
Hi Petrobras, well met 
does it worth the cost in Third Age?
really, no; except for few major cities there is little sense in having governors in villages, towns and even the majority of large towns as they won't repay themselves with enough money. A 7 chivalry general will add 0,5% to the growth of your city (not sure if in DAC has been changed though).
Generals, when ruling, still cost unkeep?
the upkeep of the bodyguards is 250 gp, the other 250 gp are calculated as wages; you can only save yourself 250 gp if you camp a general into a fort (not a settlement, but the kind of fort you build along with watchtowers), but no, general's bodyguard upkeep will not be saved if they stay into a city/castle
Should I had adoptions or only pure bloods?
that does not matter at all with regards to costs, but usually a fruitful/powerful family member will generate fruitful/powerful heirs.
if you want to learn how to train few excellent governs, have a look at this guide
http://www.twcenter.net/forums/showt...on-60-Complete
if you have questions specific to vanilla TATW post them on the Q&A thread, while specific questions on submods should be posted on the related thread