Don't rely on selenium toning to compensate for underexposed or otherwise improperly processed negatives. You should refine your exposure and development to give you the shadow detail and contrast index you want.
Then, when you have the occasional negative that needs a skosh more contrast overall, you can use selenium intensification to achieve this. You should not, however, make this a basic part of your film processing scheme!
I use KRST at a 1+2 dilution for 5 minutes (more seems to have no further effect, but it won't hurt since the negs tone to "completion") to get a Zone expansion of about one zone. The shadows are not significantly affected, rather the toner intensifies the negative proportionally, with the highlights getting the most added density, the shadows the least. Selenium intensification will not give you more film speed or compensate for underexposure.
Temperature matters in all chemical reactions: colder = slower, warmer = faster. However, with selenium intensification of negatives one usually tones to completion, so the exact temperature is not critical if you allow adequate time for the toning to take place. Room temperature should be alright unless you live in an igloo...
Note: selenium intensification does not work on pyro-developed negatives as it removes the stain, taking away contrast. The net result is about zero. There may be some way to selenium tone pyro negs and then re-activate the stain, but I do not know of one.
Best,
Doremus Scudder
Best
