I have done this myself before. I have made several one gallon batches in one gallon plastic water containers. You may want to reconsider not using a secondary though. Clarity suffers a great deal going full term in the primary plastic water jug.
Why not ferment a basic Mead in each jug for three weeks or in a single 5 gallon fermenter, then transfer them to empty plastic water jugs as secondary’s? You could place the fruit in at that stage. Then there is less risk of a stuck fermentation from the fruit messing up the PH balance of a must and killing the yeast. Do not laugh.... this has happened.
You may want to also consider using the juices of these fruits and berries rather than the whole item itself. Clarity is the reason. Fruits and berries can break down. Particles of them can become stuck in the must itself creating a cloud in your Mead. Juices mix well and do not create this issue.
The optimal would be to add the correct juices to a filler/fermenter then rack the Mead back into the filler two weeks or so before bottling.
Blackberry / Vanilla
2 Vanilla beans
I would juice the Blackberries, strain out the solids, through the Blackberry juice in a saucepan and boil it with the vanilla beans for a few moments, then add this tea to the Mead a couple of weeks before bottling.
1 Large orange (peeled)
1 cinnamon stick
Everything in the saucepan again, strain out the solids and add this tea to the Mead a few weeks before bottling.
White Peach / Raspberry
2 or 3 White Peaches
1-2 lbs. Raspberries
This one I would just juice, strain out the solids and add to the Mead a few weeks before bottling.
1 lb. Habanero's (I suggest 1 tablespoon, perhaps 2 but not much more then that)
Juice the mango, strain and make a tea of it with the Habanero's
2 Vanilla beans
Same here, Juice the Coconut, Pineapple and Orange, then make it a tea with the Vanilla beans