When to Barry's website=

http://www.barry-thornton.co.uk/

When throught the Ultra instructions-

http://www.barry-thornton.co.uk/dixactol.htm

and it says -

"Stop and fix in the normal way ensuring that the stop bath is not too strongly acid."

So you can use a stopbath as long as it is mixed a little week. Or you could switch to the milder citric acid stopbath.
The whole issue with staining developer is the tendency for acid to etch the stain developed during development.