Do we know how the whole Muse/Lord class thing works or is it up to speculation again? I've been told that a session with only two players has a Muse and a Lord but then there's "Only Cherubs can be the master classes," which is making me rather confused.

Muse and Lord are just very rare and powerful classes. It’s never been stated that they are cherub exclusive.  Only Cherubs in Homestuck hold the titles and the classes are gender-specific so that might be where the misconception comes from, but any species should theoretically be able to have. And it hasn’t been stated to my knowledge the the classes are exclusive to one or two player sessions.