Alien evolution

Once an alien has reached its "adult" chestburster stage, its body continuously begins to grow, its limbs also begin to sprout, somewhat in the way a tadpole becomes a frog. Once they have reached a certain size, they shed off a layer of "skin" much like a snake, to grow even larger.

This process more than likely repeats several times before the chestburster reaches its more familiar form, although it is initially much smaller than it will eventually become, & although many of its features are present, such as the inner jaw, they are still relatively under-developed, as shown in the chestburster scene in Alien³ (especially in the Theatrical version). The creature will continue to rapidly grow as its features become much more developed & it eventually reaches adulthood.

the alien grows from a chestbuster to an alien with no legs like a large chestbuster with arms
then becomes the alien

in the book xenomorph the chest buster would eat the host and become the alien

also if the alien is born from an alien host it would become the alpha alien(this very very very rarely happens).

Ummm, i think you said it was from a book but if not then i dont know where you got that alpha alien thing.

The alpha alien is the first born alien other than a queen in a hive. I dont even know if it is possible to impregnate an alien with a chestburster.

it very rarely happens

as it says in the book "As it begins to bust the baby burns thus causing the skin to block most bullets and stabs...the evolution takes longer but becomes stronger...if the baby does not make the bursting the babys blood burns rapidly and more strong"

so this very,very,very,very,very rarely happens

Pretty much the only thing that is considered canon are the movies. Games, books, etc. can be made into all different ways, twisting what we know. Movies have the people who originally created the movie monsters, and i guess they would/should have a part in the background of the creature.

It's pretty much a "They made it, what they said goes." kinda thing i guess.