Keep in mind though, temperature affects speed of toning. Using time alone may not work very well for you.
If you are using FB paper, saturating the print with water first is very important. Otherwise, selenium toner will be absorbed in paper. Not only is it wasteful, it will be hard to wash off completely. I am not sure about RC. I'd dip it in water first just in case.
Yes, it is VERY important you wash WELL after selenium toning. It actually contains large amount of chemical that is also an active part of fixer.
My experience is that MGIV of any kind is very resistant to toners. Using 1:20 of Kodak variety, I've got it to darken slightly but not that much. Take some junk print and cut it up into pieces. Try it at 1:20, then 1:40, then 1:120 by successively diluting with more and more water. With slow speed of "toning" you may want to use stronger solution even if your goal is to get Dmax change only.