I have an 8400. Perhaps the setup is the same since it is essentially the same machine (at least that is what Xerox tells me- a little faster and different ink formula.)
Using Windows? If so, Start/Control Panel/Printers. Rt Click on your xerox. Properties... Printing Preferences. Paper Quality tab. Force Printing from a Specific Tray.
But, this makes my 8400 always prompt me for the paper for Tray 1. You mentioned an HP laserjet, which I have and I remember how if it senses paper in tray one, it will feed from that regardless of the size you need- which stinks if you just left some nice paper in tray 1. I don't think the xerox works that way. It is a good bit "smarter" than my old HP. It will consider your page size before just using up your good paper! I think that even if you close the tray, it will want you to feed from Tray 1 (or whichever tray specified).
Now if you use the same size paper all the time, ie. letter size, then you are in good shape and it will only select from the tray you specify. You can leave it empty and let it prompt you for paper, feeding it basic white or nicer letterhead with more ease than opening the bottom tray.
Nick