Though the abilities of many of the quasis runs contrary to the RAW definitions of the elemental subtypes, I think it is far more important for style and thematic reasons to keep the respective subtypes with each type of quasi (ash/fire, vacuum/air, etc.). It should be easy enough to simply put in a line or two of where the contradictions in each creature lie. For instance...
Fire vulnerability: Unlike most creatures with the Fire subtype, ash quasi-elementals are vulnerable to fire.
Though this seems to disregard the rules, I think it is a good policy to assume that a creature's individual description supersedes that of its type and subtype. Just pretend there's a "unless stated otherwise" on all the subtype descriptions
I just think the elemental associations inherent in these creatures' descriptions and origins are too important to ignore.
Fire vulnerability: Unlike most creatures with the Fire subtype, ash quasi-elementals are vulnerable to fire.
Though this seems to disregard the rules, I think it is a good policy to assume that a creature's individual description supersedes that of its type and subtype. Just pretend there's a "unless stated otherwise" on all the subtype descriptions
I just think the elemental associations inherent in these creatures' descriptions and origins are too important to ignore.