The 600 coin females and 900 coin males will award pirate coins for freeing them (or the reward chest, which can contain the island soul, sailors, or 90 percent of the time (estimate), hp potions (bound i think) and very small amount of silver. Most of the time, freeing the slaves is profitable and will net you more pirate coins than you spend. The other prisoners cannot give you pirate coins, they only reward like 3k-10k silver, althoug you can get the island soul and complete unas tasks from them. If you just want to complete the task quickly/get the soul you can free any of the slaves since sometimes youll need to check back several times before you find 5 slaves of the type that awards pirate coins…I have seen maxroll guide claim that 2 of the slave types give providence stones, but as far as I have seen this is not correct in our version of the game, and the slaves give either silver or pirate coins. Someone can correct me if they have seen some slave types give p stones in western version. Note you can save the release certificates to hand in for unas tasks on the island for leapstones later.