Just killed the ones in the Great Hollow and got two Red titanite Chunks

There is a specific one in the Great Hollow which drops them. If you really need some, the Black Knights in Undead Asylum drop some, and you can trade with the Crow

No. They are unique enemies and do NOT reappear, none of them.

They do reappear if you don't kill them when you first run into them but they do NOT respawn after death.

Yes it does , most important in the great hollow because you may get a titanite Slab from the last lizard you kill . Equip sliver serpent ring with 10 humanities . With 300 item discovery - chances are above 90%

gold serpent ring*. Silver one is for more souls gain.