Arlong never was a warlord. They simply presented him as the strongest pirate in east blue...

Does Arlong become warlord?

No, during the Arlong Park arc it's mentioned that Arlong was released in the East Blue by a Warlord called Jimbe but it's not talked about more than that. That's the only relation Arlong has with warlords and he isn't one himself.

Who is the weakest warlord?

Out of all the 7 Warlords that fans have been introduced to so far, Buggy remains the weakest. He isn't particularly good at anything, and his Devil Fruit isn't all that impressive either.