Maybe just because he plays the title role in the opera The Minotaur, but John Tomlinson feels like a good fit for Asterion's singing voice:
but he does have a real good low voice-- warm and resonant, rich but gentle. Suits Asterion.
P, Argos, and Luke are definitely tenors. Kota, Storm, and Khenbish probably baritones. Themba is another bass. And the cobalts all altos/countertenors. But I can't think of any direct reference-points for them. Haha
(And be sure to get an Aulos too, so it can be like the music that accompanied Aeschylus/Sophocles/Euripides plays (which were themselves musicals or operas))