As for limiting research to Pures, that seems wrong to me. I did suggest in RMU Spell Law that the GM might want, for example, to limit Closed List research to Pure casters (though that is not a core rule, but an example of one possible house rule), but I would think that the best person to research a Ranger spell is a Ranger. One can affix all sorts of specific limitations such as a knowledge of fifty existing spells before attempting new research, to have an adequate general background knowledge of spells. It all depends on the GM and the setting. Spell research might not even be possible in some settings, all spells having been passed down by the gods.