I wouldnt do it to begin with, because at best it would include Lila and Helga.
If you want to strench a little, include Rhonda or some other girl (not Ruth or Phoebe, this is stupid), but you would have to work on it first to be sure that everything is in character. Also, follow the tone of the show, which probably means that in the end nobody gets with Arnold and he didnt wanted it to begin with.
If your porn isnt going to look like canon, or at least behave like canon, why even use these characters to begin with? Make an OC.