Cordova,
The new Floor Rules that will be updated in January will expressly deal with this.
The custom requirement will be that you have to have the original pieces card to play a custom of it.
So if you bring the original Rex card and a custom Rex then you would be ok.
As far as you guys playing DCI. Do you play at a Store???? Do they run other DCI games such as MAgic?
If they run Magic then your Store owner can getyou set up rather easy.
If you are playing at a public venue like a library then you need a TO and 4 players.
You can have someone register to become a TO at Wizards DCI website:
http://www.wizards.com/Default.asp?x=dci/welcomeFrom there you can become a TO and order DCI cards. Once you find your TO or get registered as one get back to me.
I can walk you thru a bunch of the stuff you need to do to get you guys up and running.