Would it be unreasonable to perhaps use multiple_legs where the additional legs would be normal (such as spider girls, insect girls or monsters) as opposed to having an additional set of legs (ie "extra" like in post #10476 and Okuu's 3rd leg in post #939212)?
Condensing what I'm suggesting:
- multiple legs = the additional legs are normal on the body
- extra legs = the additional legs are abnormal on the body