the old fashioned way is to use sugar to pulp at a ratio of 1:2. Two parts sugar to one part pulp. If your fruit is very ripe, then you need to add lemon juice or some underripe fruit to raise pectin levels. I find that if i'm going to make jams the old fashioned way with the modern twist of pectin, that it works best to cook sugar and fruit for a bit until it has reached almost soft ball stage ( slides slowly down a chilled plate) and then add the liquid pectin according to the recipe amounts included as to how much pectin to use. I mix fruit all the time and am successful.
Plums and blueberries and blackberries with allspice, cloves, cinnamon and grated fresh ginger makes a great combination.