I am not sure about the Kodak Powder fixer, it's perhaps just a non-hardening powder fixer with fixing times from 5-10 minutes(?), while rapid fixer is less than 5 minutes.
Usually, you'd use a hardening fixer for certain films (Efke, Foma), these types of fixers usually have a fixing time from 5-10 minutes, so using a regular non-hardening fixer which is "slow" seems a bit mute in my world.
I would use the rapid fixer for TMax films.
There may be differences in how many films you can fix before the fixer is exhausted as well, regarding "slow" fixers and rapid fixers, but I don't have an overview on that.
2. Sticky threads are "glued" to the top of the forum head, so that they are easy to find, regular discussion-threads disappear downwards when they get older and other threads are created, sticky's stay on top. (usually these are important thread or thread that contain information about the most asked questions in a forum).