Steeda tri-ax
 

Will only factory and steeda shift knobs fit the tri-ax?

Your question is a little confusing. But, as far as I know all the after market shifters use the same thread size as stock. So basically, any shift knob will work if it says it will fit a Mustang. So, I guess the answer to your question should be "no" as other shift knobs should fit the Tri-ax as long as they fit a stock Mustang shifter.