My XL750 is on the way and will be all set up for 9mm, but I was unable to find a Dillon .223 die set in stock. Got the .223 caliber conversion kit, but no dies. I'm probably going to snag a separate Dillon seater die and crimp die, but for sizing, my question is:

Can I use the RCBS X-sizer die I already have in the Dillon toolhead? Would I just thread in the RCBS die and adjust according to the normal Dillon process?

Yes, it will work fine. I have RCBS, Lyman and Lee dies in my Dillon.

Oat Action Man - You are about to ask yourself in about 3-4 weeks one simple question - Why did I wait so long to buy a Dillon (assuming you do not have one now)......I did when I got my 650 several years back.....Enjoy Sir...I LOVE my time reloading on my Dillon press especially when you get everything set up and it is running flawlessly......Mark

 
I use Dillon and Redding dies in my XL-650. As long as the dies have standard 7/8" x 14tpi threads, it will fit the press.