However is always followed by a comma. Examples.
Mostly is almost always followed by a comma. Examples.
Rather is 50-50, I think. Examples. (I think this is the case with so too.)
Mainly is more often written without comma. Examples.
So my question is, how to tell which introductory words always require a comma and which don't? Also, do they belong to different categories?
I refer to these type of examples:
"Mostly the population is concentrated in the Zhovtnevy and the Prymorsky Raions."
They decided not to put a comma there ... but I think you could put a comma there also.