well illegal is illegal, but so is putting an aftermarket air filter on, so I won't start on that one.
My advice, and I know it is not worth much, is to get rid of the 6x9's and find something that fits. Don't go cutting up a car just to fit some speakers in. There are pleanty of other ways to get the sound you want without stuffing up your car.
If you really want the 6x9's in your celica go with what joel suggested and stick them in the rear side panels. I am not sure about the 23's but the 28's have a big hole there. I had a 10in. sub in each side at one stage, so some 6x9's will certainly fit.
As far as stiffness goes, the celica's aren't too stiff to start with, so why make it worse.