It's highly debatable whether "các" and "những" are used interchangeably. http://bit.ly/2jYxFfz (Links are in Vietnamese only, sorry :p). Being a native Vietnamese speaker, I would say that it is safe to use them interchangeably most of the time. However, there are cases that you must choose only one of the two words; and knowing the context helps a lot too.
As of your question, I would say that "(THE) chickens" can be translated as both "CÁC con gà" and "NHỮNG con gà" no matter whether the chickens are general or specific ones. It's really difficult to differentiate between the two meanings without context. :(