Kalki's story covers that phase of Arulmozhi's life when he was not even an 'ilavarasan' and had no title whatsoever. Of course, Kalki indicates several times that Arulmozhi has the potential to become a famous ruler, in due course. Hence the title "ponniyin selvan" is more appropriate than the titles inherited by Arulmozhi later in life.
Thanks Sivapathasekaran, Shankaran and Sampath for your responses. I really appreciate them.
And Sampath, you are right in one way. Raja Raja may or may not had any titles during the period covered by PS, but I am still wondering what made Kalki to choose this particular title. When writing about great people, usually everyone get instantly attracted when given a famous title which is known by all. But Kalki daringly chose a title with different aspect compared to all other titles of Raja Raja. May be he wanted to give a unique title which stands out from the rest, but I am still awaiting for a good clarification.
Just a thought. Kalki at more than one point, emphasizes through Kundavai that Arulmozhi was called as 'Ponniyin Selvan' as he was saved by Ponni/Kaveri herself in his young age. Also, Sundara Chozhar also remembers that event as a proof for Ezhathu Nachchiyar's revenge-seeking acts towards Sundarar's children. The same incidence, from Ezhatharasi's point of view and Arulmozhi's point of view, shows the love she had in Sundarar which turned towards his children. Thus the same incidence gets discussed again and again by various characters in the storyline and also happens to be one that gave Arulmozhi a title. Kalki would have thought of this title to be more suitable for his story, which connects Ezhathu nachchiyar to the story through the incidence, As her relationship with Sundara Cholar was the triggering point for the complete story if we analyse.
Remember how kalki explained his naming of the chapter 'Thyaga Sigaram'? Comparing with that reasoning, i too think that due to above mentioned reasons 'Ponniyin Selvan' is the best suited title for the story.