How are you filling and packing them?
If you're just aiming to do a cold clean fill into PET then you'll need to keep them chilled, and 6 months is highly unlikely to be feasible.
Some ascorbic acid will help with oxidation, but it's not all that common to use for pineapple or mango as they tend to be relatively robust juices, and if you're getting significant oxidation problems you may want to look at your process. Heat tends to increase rate of oxidation, so in a cold-processed product it is a bit unusual.
You should fully expect oxidation to continue once packed, as this is often one of the limiting factors on shelf life for some types of longer-life juice products - there will be some dissolved oxygen in the juices, and many packaging formats aren't completely impermeable to oxygen either.
Stability for cold-pressed juice is very challenging, as there is no thermal deactivation of the natural enzyme content and this can exacerbate the visual problem of the pulp settling. Some pectin may help with this, but it is unlikely to stop it completely and you'll need to check the regs in the country/countries you'll be selling them to check what additives are permitted.
Reducing the pulp content will also help slightly with the appearance, but you can't completely avoid it without engaging in heavier processing. You may want to consider using this as a marketing device along the lines of "Our juices are naturally pulpy and will settle over time: Please shake well before use", as it does at least avoid people having too much cause to complain about. Won't stop the complaints completely though - you'll always have a background level of people who want to eat natural products but don't like the fact that they do natural things 
