Sheldon: "English and Italian are all semi-interchangeable, but you shouldn't go back and forth between different types of freewheels on the same hub repeatedly."

Apparently Surly even issues the same warning: "It is important to not switch back and forth between cogs and/or hubs with different threads." (http://surlybikes.com/uploads/downlo...og_5.16.05.pdf)

I personally would not advocate mixing the two threads, but go ahead if you must.