As for Iris's design, sure it's original, very unique and the monster even looks beautiful at times, however does it ever give off the vibe of scary, or fearful? I never got that feeling from Iris, or Legion for that matter. The only monsters in the Gamera trilogy that might instill fear is probably Gyaos, or Gamera in the final part maybe. This Godzilla keeps being referred to with words of nightmare and fear, but looking at the past monsters I didn't get much of that from Mahiro, and Iris was the perfect monster to create this feeling with. The Gamera killer, a monster that was made to give kill the only monster that could stop the Gyaos, but while it is a beautiful unique monster, it didn't fill my idea of fear.
Mahiro also does work for animation and I haven't seen all his work in that but Escaflowne, and Evangelion. Interesting designs once again, but don't really fulfill the role of fear, especially the majority of the Angels who were supposed to bring us death.