Larisa was sparked by the idea of Klodi Monsoon as a PB, so I can't really imagine anyone else in the role. As a male, Barney StinsonNeil Patrick Harris all the way.
Peer Pressure: Miranda Kerr was a good alternative. As for genderbent... Missy got older. Josh Holloway as genderbent.
Genderbent forms for the rest of them oh boy this feels so wrong:
Cassandra: Hugh Dancy (our Morpheus), no doubt! Pandora: Gerard Butler. Don't ask, I have no idea. Raphael: Delta Goodrem Rosier: Crys had figured out a great PB several months back, I'll have to poke her about this. Sleipnir: Candice Swanepoel (Beroe; there are virtually no images of her clothed in more than lingerie, I swear...)
As for the others, I may have to come back to this later.