See as a fellow Taric enthusiast who also happens to be a Warden I can answer that question. You see magical girls have a couple qualities: They are fabulous, star themed, and were gifted their powers.
At first glance Taric seems to already be a star guardian. He is fabulous, he literally wields crystallized starlight, and he got his powers from an aspect. However there is an important difference. You see Taric didn't just get his powers from his Aspect, the Aspect literally became part of him.
Clearly he is not a star guardian, he is the one who leads the star guardians, the one who gives them their powers per say. As such his choices even make sense:
First he recruited Lux, because he is clearly following the logic of "Keep your friends close, but your enemies closer." He has to keep a watchful eye on her so she doesn't start dating Ezreal.
Next comes his first mass recruitment. Janna, Poppy, Lulu, and Jinx. Janna, Poppy, and Lulu are all kindred spirits in a way. Taric would probably get along well with all of them. Jinx was hinted at long before. Taric has a special interaction with her "Shine on you crazy diamond." He was clearly referencing how he would make her a star guardian.
Now comes the second set: Ahri, Syndra, Soraka, Miss Fortune, and Ezreal.
Syndra Miss fortune, and Ahri are there to make the playerbase focus on them so they ignore him sneaking Ezreal in. Furthermore Ahri may possibly have been included to steal the memories of Lux to kill that ship off. Memory loss being a common theme in Magical Girl shows no one would bat an eye at it. Soraka comes in on the same terms as Janna, Poppy, and Lulu.
Thus Taric cannot be a star guardian because the star guardians were created by him, the man who's natural power is the power of the star guardian skin line.
