Maybe you can use Persons and Orgs modules which can have different set of fields instead of using one module Persons and try to separate fields there ? and you can set different settings for default membership level in Persons and Orgs modules. 

Such setting will potentially cause many dependant problem and it's hard to explain it's meaning to end users, so it could end up with user will not use it properly.  Or you can ask someone  to develop custom module for you.