it's rated for between 40 to 120 gallons so i don't think that's overkill a lot of people put a bigger skimmer on their system because as time goes by they upgrade and put more stuff in the tank here's a video to better explain http://www.youtube.com/watch?v=MLAgZopwyDM
The recommended pump for that is a mag 5 or similar. I think that skimmer is fine on a 90 but the pump is probably overkill.
I have the same skimmer on a 75 and it runs fine.