Rudi,
Like I posted earlier, I transfer prints directly from the second fix to the selenium toner as a standard procedure. Yes, it is the second fix and has less dissolved silver in it than a first fix would, but I suspect that there is a certain amount of carry-over of dissolved silver in the form of silver thiosulfate complexes into the toner. This seems to precipitate out as black silver selenide (or something similar, haven't tested this, just surmising) and is the precipitate that I filter out before and after each use of the toner. I would guess that it reduces the toner life somewhat, but not appreciably (and not having to spend time on an extra wash step is worth it). As long as the fix is adequate and the fixer not too acidic or exhausted (and by this I mean no more dissolved silver than allowable for a second fixing bath before it gets promoted to the first fix, so somewhere under 0.5 g/l), there should be no problems. Ansel Adams advocated transferring prints directly to the selenium toner from a second fix that was a plain hypo solution. I have used plain hypo, TF-4, and Ilford Hypam and Rapid Fixer in the 1+9 dilution as well as Kodak Rapid Fix in the "print dilution" immediately before the selenium toner with excellent results. The only time I ever got staining was when the second fix was inadvertently not changed out and exhausted. This resulted in a yellow overall stain that disappeared with a refix and a wash. Residual hypo and silver tests are good on prints toned in 10+ year-old replenished selenium toner.
And, although KRST contains ammonium thiosulfate and the freshly-mixed toner has a distinct ammonia smell, the thiosulfate in older replenished toner must be very little and there is no ammonia smell at all. Nevertheless, the toner works just fine and just the same as fresh toner.
So, at least for me, there are zero problems with the toner that result from transferring prints directly from a second fix or from reusing and replenishing the toner.
Kilgallb,
Staining of prints in selenium toner seems to be the result of a couple of things. Worst is incomplete fixing. Transferring prints from a too-acid solution to the toner also can cause staining. Partially washing a print will cause mottling due to uneven toning. There's something about the remaining fix and/or unwashed out silver complexes that affect toning rates. However, if you transfer directly from a neutral or alkaline fixer that is not exhausted (second fix, see above), then toning is even and staining does not occur. The alternative is to wash thoroughly before toning. Just rinsing or washing for too short a time will cause problems. I find the time saved by not having to wash before toning way worth using a neutral pH fix and keeping an eye on my fixer throughput.
Best,
Doremus