Yes, rebuttal is ok.
Essentially the 'fair play' of the final post is as not to keep hidden scans/arguments up to the last post, just so that it cannot be rebutted [ther's been quite a few posters who've done that over time]. So Coldcast suddenly making a Black Hole or Absorbing Man transforming into psionic energy is not an argument that should be made there.
You've both already done your 'moves'. At this point, it should be the why it works or doesn't.