"put down rebellions,"

[Fyron]

Alneyan said:
1) Your homeworld cannot go below Angry. If you are playing with more than one homeworld, the others might riot, or not. I am not too sure here.

Only the primary homeworld is immune to rioting. All other "homeworlds" count as colony worlds. There really isn't an easy way to determine which HW from a multi-planet start is the primary HW. It might also be immune to some planet-affecting events, such as planetary core instability. It is not immune to being destroyed by events such as star destroying ones, however.