So Im brainstorming with the Cities of Sigmar and am wondering if it is possible to get the Hydra, Corsairs or Chariots as Battle Line somehow.
You are not familiar with pages 127/128 of your CoS book, are you?
Reading those two pages + the Keywords on the bottom of each unit will answer these types of questions.
also wondering if the start collecting box comes with the option to make both the scourgerunner/cold one chariot and the Kharibys / Hydra kit.
Yes, it's the same kit. You just use different pieces off the sprues.
All of the multi-kits are like this. GW
just lists them under each name on the site so you can search by the various names.