Can I disable Sliver Overlord's abilities with Pithing Needle?
Sliver Overlord's abilities are not mana abilities. Mana abilities are abilities that produce mana, like the ability on Sylvan Caryatid. (There are a few restrictions; not everything that produces mana is a mana ability. See rule 605.1 if you're curious.)

Sliver Overlord's abilities are simple activated abilities, with mana in their cost. Pithing Needle will work just fine against them, as will anything else that prevents abilities from being activated (e.g. Arrest), and anything that makes it lose abilities (e.g. Lignify).