Introducing Earth's species to a terraformed planet would help stabilize the ecosystem. It is theorized that animals would travel on space ships similar to humans. they would be bred on the ship and then released onto the newly terraformed planet. Over many generations they would adapt to the environment.
Reasons why it may be beneficial Edit
Introducing terrestrial species to an alien planet would help keep the atmosphere stable, as trees and other plant life.
After oceans were formed on the planet, the next step would be to add hydrogen, oxygen, nitrogen, carbon, phosphorus, sulfur and methane. After that, we would use devices to create high energy forces, like electricity, to bind the elements. Then, simple, single-celled life will develop and evolve into more complex life.