I would use 5- 6lb of rhubarb per gallon, defrost in the bucket for a few hours add 4 pints boiling water stir and crush , cool add 2 teaspoons pectolose per gallon. Stir and crush for three days then run through muslin twisting to remove max juice , you can work out how many gallons your going to make , so say you have one half gallon of juice and water, you can add your sugar dissolved in a couple of pints of boiling water. then top up to a couple pints below 2 gallon (you need a space in your demijohns)with either water, white grape juice , grape concentrate or apple juice. Then add i teaspoon of yeast nutrient per gallon and a german type, low alcohol yeast. ferment for a 3-4 days in bucket then transfer to demijohns to complete fermentation. acid can be adjusted before bottling with citric acid powder. i would use two and half pound white sugar per gallon reduce slightly if sugar in any juices you use. taste when fermentation slows , if too dry add a little sugar syrup, you can sweeten once it finished fermenting and been racked a couple of times with a non fermentable sweetener like Truvia, be careful its strong stuff.. I would make crumble with the plums.