When a girl starts to have period, would it really decrease growth rate and normally how many more cm can she grow? Would the hormone reducing injection help to boost growth again when it's done after period starts?

Your growth is all in your genetics.
Look at your parents and grandparents heights, odds are you will be similar to them.

If you parents are short, you will probably be short.
If your parents are tall then you will probably be tall.

Yes, growth does slow after puberty. Do some research on the internet.

It also depends on when the period starts. If the child starts what is called precocious puberty... periods before age 10, then yes, it will cause her to be small of stature. There are cures for this.