What does that bag weigh? Here is a recipe for you but youll have to divide the batch by the amount of fruit you have. You can still use 1 packet of yeast though unless you have another batch of wine ready to be made then you could divide the packet if the other batch is also smaller then 6 gallons. By the way, welcome to this site. http://www.winemakingtalk.com/forum/showthread.php?t=3500
i think the gallon of plums would weigh around 5 or 6 lbs.
side note----picked some wild blackberries this afternoon, they were used for a cobbler too hot and dry, the blackberries do not have much juice in them.I wanted to pick some to make wine out of, maybe next year if we have more rain.