Matt, whether or not selenium toner is used, the fixing time in Ilford's optimum permanence sequences should not exceed 1 minute. Also... it's
ammonium thiosulfate in rapid fixers
Jessestr: Regarding mixing the selenium toner with Ilford Washaid, I would follow Ilford's directions on this
if following their optimum permanence sequence. Even though a fresh Washaid treatment follows toning, the use of Washaid to dilute the selenium toner likely speeds the eventual washing. In addition, Ilford's Washaid provides a weakly alkaline medium which seems to help prevent staining when going directly from the fixing bath to the selenium toner. If you decide instead to mix the toner in plain water, I would follow Matt's advice and do a wash between the fixing bath and toning bath.