Question

Can I get ralts with her hidden ability (telepathy) in Pokemon ruby? (not omega ruby but the old version of the game)?

Answer 1

Telepathy (Japanese: テレパシー Telepathy) is an Ability introduced in Generation V.

No, you cannot get any Pokemon with Telepathy in Ruby/Sapphire because Telepathy was introduced in Gen 5; Ruby/Sapphire is Gen 3.

Source