I actually think Astarion's story interacts with the tadpole in a really interesting way. The tadpole IS his means of escaping his vampiric master. It isn't so much two terrible things happened to him at the same time. More like he's been a vampire slave for 200 years and this new thing that has happened to him opened up an entirely new path for him that might let him regain his freedom.
In that way Astarion works with the tadpole plot better than any other companion aside from possibly Lae'zel. Shadowheart's story might tie into it later as she discovers the mind powers might give her an edge as a cleric of Shar or perhaps is an avenue to reclaiming her memories, but that has yet to be seen.
Wyll and Gale are the only ones who's stories are entirely divorced from the tadpole plot and don't have an obvious way of integrating them going forward. Though who knows? We've only got the first chapter of the book, after all.
