James, why did you move these specimens from P. floria? That is the species most commonly encountered in this part of the country. Although these two species are difficult to differentiate, the morphological characters on these seem to match P. floria rather than P. pruinosus. Did you notice something crucial for the ID of the latter?

I thought I left a comment when moving, apparently not.

I've had some discussion with other isopod enthusiasts I know in person, as well as a few experts, and we've long been searching publications for information on this genus. This topic has been going on for several months. So far the only reliable difference that seems to stick when comparing known specimens is the antennae banding. For instance: http://static.inaturalist.org/photos/2419743/large.png?1442970477

If you know any other morphological characters that are reliable, please advise!

According to what I've been told (unofficially), pruinosus is somewhat frequent in southern CA. In any case the companion species of P. pruinosus are VERY common in California, so this species probably is too. That isn't to say P. floria isn't here, it almost certainly is too, but I think both options are at stake here rather than just floria.