0, you don't need SP.

You need whatever token you are trying to suppress rewards for.

There is no answer to that question because each Tribe has its own tokenomics.
For example, STEMGeeks only produces 840K tokens a year while other Tribes produce as much as 10x that. While 1M works fantastic for STEM and will for the foreseeable feature, it won't for other tribes that are producing way more tokens a day.

If you or someone is planning a tribe, I highly recommend spending a good amount of time on the numbers as they are completely arbitrary on their own but once you put them together have to work in harmony.