Yeah, I mean, I saw what @CG_CapGaSP wrote - and it's all up for interpretation. I think any KOTOR fans/players are probably more free with the label, as from that point of view what we have now is more a set of guidelines. If you're JUST going by the movies then yeah, the rule of two, and you're Sith roster is going to be shallow.
Still, I'd give it that when it came down to it, Ventress was first a Nightsister and then a Scoundrel/Bounty hunter as the show went on - which is where she was left.
But Maul/Savage don't need to be female to be labelled Nightsister Allies - speaking of which, I wouldn't mind having Spider Maul from the one episode - you don't get to choose your targets with him, but he's devastating....
Throwing in Mother Talzin - so she isn't an Old Daka clone, maybe instead of resurrecting a toon, she replaces that slot with taunting shadowy undead night-sister ninja's - if she out lives her team she could zerg for the win, assuming the enemy has been weakened.