Does 'no more room in the Inn' mean spare room not big enough for a birth? In this article, the author says that 'no more room at the Inn' from Luke doesn't imply a Stable. 

The big idea is that "there was no more room at the inn" from Luke is misread, and instead of Mary giving birth in an animal shelter – the text means "there wasn't enough room for a birth in the spare room, so they took over the main room". The author suggests that the reading of an animal shelter perhaps is a gnostic gloss. 

My question is Is the stable essential? (The implication being, that if he was born in a Stable, then he is born as one of the poor, and if he was born 'in the main room not the spare room (Inn)' then Jesus is born as one of the middle class).
Since it is not in the Bible stable is not essential. But your implication of it is wrong. Just because Jesus was not born in a stable, it does not mean he was from middle class.

First, Middle class did not exist at Jesus's time.

Secondly Luke 2:7 says that he was laid in a manger. Some scholars claim that in those days families had animals inside their home to protect them in winter. It is very unlikely they were kept in the main room. So for Mary to have access to a manger she should have given birth in the portion where animals were kept (this may not necessarily be a stable).

It is not a usual practice to place new born in a manger. And that is why it is given as a sign for the shepherds. Also the host lives in the main room, so a pregnant women who is expected to go into labor anytime would not have any privacy in there.

It is very logical to assume that Jesus was not born in the main room but where animals stay.

Finally think about this: God has eternity to plan his birth. But he choose to be laid on a manger. That is enough sign on the importance he gave for poverty.