If it were me, I would staek the mangoes. I would also cut the Nam Doc Mai back and allow it to start branching low (Harry - I know you disagree with this). Zach - if I remember what you said, you would rather tyou plants be shorter and wider as opposed to "free growing" with no height restrictions. To accomplish the shorter (more manageable) and wider trees, you will need to start topping them (there are many threads here about topping, or "pugging"). By topping you will also promote growth as well as branching.
With respect to the Jak, I would leave the fruit to maturity. The tree is definitly large enough to carry it. Once you harvet the fruit, you can them begin your topping and branch promotion. Again, if I am correct, since you want the jak a more manageable and wider tree, leaving the fruit on and temproarily "stunting" the tree might be in your advantage. You could cut the fruit off but I don't think it is necessary.
Bottom line. there is no right or wrong here, only decisions you need to make based on what you want and what you want out of the trees. In any event, welcome to the addiction and good luck with these adn any and all future tropical fruit endeavours.