Hey JJ-
I know a 90g or a 120g are not big enough for most puffers. I know you said you liked porcupines so I'm assuming that's what you're thinking about. I got mine when he was just a little guy and he grew at least 7 or 8 inches the first year. He's now around 13-14" at least. My 300g is not big enough for him, stop by sometime and take a look at how he dwarfs the tank and then picture him in a smaller tank.
And they do eat corals and will rearrange rockwork too. Mine dove right into some zoanthids and has had a bite or two out of a couple leathers as well. He loves silversides, but rarely gets them but has left the smaller fish alone. He certainly has had ample oppotunity to eat them but never has, maybe I'm just lucky, I don't know.
Don't put them in acrylic tanks, they scratch it up in their haste to eat. When he sees me coming with food he rubs his mouth up and down the sides of the tank and scratches the heck out of it.
Janna