I do nor like the heavy syrup as is in the fruit cocktail ; but once it is mixed with a lot of different fresh fruit a few drops of lemon juice, or even some vinegar will change the flavor to my liking.
I use canned fruit cocktail as a base just because my wife likes it. Then I add fresh grapes of different colors , a variety of diced melons ,diced red and green apples , diced RIPE pears (if there are no ripe pears I'll do without them ) and finally all the fresh berries I can afford.
There is really nothing that will stop your Guacamole from turning brown. everything mentioned only delays this discoloration, The best thing to do id eat it right after you made it : but then it turns brown too.