Last time I did a test build on their website, it automatically changed the rear wheel size based only on the suspension selected. I just went and did an MX 1.2 build again and they have decoupled suspension and wheel tire selection. Meaning, on an MX 1.2, you can order MX suspension or Enduro suspension. Then, when you go to tire selection, you can select MX tires and get 21/19 wheel/tires or you can select Enduro and order 21/18 enduro wheel/tires no matter which suspension you chose.
Though I don't know why you'd order 21/19 with Enduro suspension or 21/18 with MX suspension, it's great they allow that much order customization. And then choose your spring/valving for rider weight on top of that. Love it!
On the EX, you can select either suspension (MX or Enduro) but the tire choice (and presumably wheel size) you can not select and so seems locked into 21/18. Maybe that will change in the future, though again, not sure why you'd want MX wheel/tires on an EX.
@fe501tired So to answer your original question, yes you can order a MX 1.2 with 18" wheel/tires and with either MX or Enduro suspension. Hop on their website and do an MX 1.2 build to see your options.