I'm sorry, you're not going to like this...
toning for permanence with minimal color change
I personally think that concept is a red herring. Toning for permanence basically entails linking the silver together with something else intentionally in order to prevent the same thing happening over time unintentionally. Hence, real 'permanence' is only achieved if all the silver is bound, and that means toning to completion. In a sulfide toner (or just about any toner, really), this means a distinct color change.
To demonstrate the above, take one untoned print, and one print that's so-called 'toned for permanence' (either selenium or sulfur/sepia) without a very clear color change (i.e. only very lightly toned). Chuck both into a ferricyanide bleach bath. You'll see the untoned print bleach back entirely, and the other will only have slight density left in the deepest shadows. The latter is the gain in permanence; over time, any environmental influences that might degrade the silver image will leave only those remaining tones in tact and affect all the others. You'll see first hand that the whole endeavor provides only a marginal gain that has no real-world advantage whatsoever to the permanence of the print.
But look at it from another angle: elementary silver as you'd find in a typical silver gelatin print is fairly stable as it is, provided the print is (1) thoroughly fixed and (2) thoroughly washed. What kind of 'permanence' do you want to achieve on top of this? Against most environmental factors, the paper base itself is actually substantially less protected as a matter of principle than the silver image itself in a well-processed but untoned print - think of humidity or fungal or insect infestations. Yes, your silver image may tone in sulfur-rich environments - but a polysulfide toner would do the exact same thing to begin with...
So in short, if you ask me, the whole concept of this approach is fundamentally flawed and perhaps one of the most re-iterated urban myths of darkroom photography.
I like the slight color change away from olive that I obtain with 1:20 KRST.
Now that's a useful application of a very light toning. I very much agree with this and that's the only reason why I tone any prints that might end up on display in one way or another. Usually with selenium, sepia or a combination thereof. Especially with warm-tone papers this really brings out the full potential of the image, IMO.
Does this toner produce dangerous or noxious fumes?
Yes to both, but more noxious than dangerous. Polysulfide toners tend to produce a distinct rotten egg smell that is dangerous in high concentrations, but you'll probably have fled the room and had a major row with your housemate(s) long before a level like that is achieved. For sulfur toning, a thiourea 'no-odor' toner is IMO a better choice, also because of its flexibility in image tone (ranging from yellow tones to chocolate browns).