Does my baby need to have teeth to eat cereals?

Nope. It's the same as edentulous (toothless) adults eating. They can gum the food. Best stick to soft foods that are easily "gumable". Almost all of the baby foods account for this.
No. No. The first 8 teeth that a child gets in the first 15 months of life are not for chewing anyway. Cereal, pureed fruits and vegetables, and then soft finger foods can be safely ingested before any teeth are present. Ask your doctor about how they prefer to progress through foods in the first year of life. Teeth are not necessary for starting cereal!
No. As long as the cereal is a softer texture it should be fine. It may be worth discussing your childs nutrition with their doctor to set up a good timeline to introduce new foods.
Teeth not necessary. Babies can eat softer food without teeth. Many adults eat everything without any teeth.
No. Babies can eat baby foods and baby cereals, properly prepared so not to be too thick, even if the babies have no teeth.