Should I go for the viewpoints feathers paintings etc before I start the storyline or after I finish in assassins creed 2

Viewpoints are a good idea as you progress through the game because in combination with treasure maps and quests they allow you to better plan out what you're doing and understand where you stand- or make the game easier, really.

Paintings increase the amount of money you get in your regular payments from the villa, so this one depends on how fast you want to beat the game. If you play at a slow pace it's worth it to buy the paintings right away because you make the money back in villa payments and can use it for better weapons and other items. If you play quickly, you might just want to skip it and save your money for weapons.

The feathers are the most useless collectible in the game. The reward you get for them is a cape which makes you completely notorious at all times and a small cut scene. I wouldn't bother with them until you're done.

Near the end of the plot, you're forced to collect all of the codex pages to continue. Considering you'll see them all on your map sometime before this, you might as well pick them up as soon as you find them so that you don't have to go back to all of the cities and hunt them down.