Um, no. You don't ever have to be cruel to Fenris or Merrill to get them to full rivalry. Consistently supporting mages pisses Fenris off enough to get him to full red bar early in Act 2, and with Merrill all it takes is criticising her use of blood magic and maybe not helping her get the arulin'holm if you're not over the line yet. Supporting mages will also get you an easy full rivalry with Carver. With Isabela, being moral and not stealing things or helping poison merchants generally gets you rivalry points. Sebastian can be a bit trickier to get to full rivalry if you're nice but still tell him to take back Starkhaven, but only because he arrives in the middle of the game and gives you friendship every single time you rescue a kitten from a tree. With Aveline, you just have to break the law at any opportunity, which doesn't necessarily involve being a jerk.
The only instances where you have to consistently be an arsehole to get to full rivalry are Varric and Bethany. (It's actually impossible to get to full rivalry with Bethany anyway, in the vanilla game at least.)
I think we have a very different understanding of the word "cruel". "Just supporting mages" was never enough for me to get full rivalry. I had to add supporting slavery on top of it, not to mention be very rude (I would say cruel) in personal dialogs with Fenris.
And with Merrill... My God! You are not "criticizing" her believes, you are crashing them. And with such a horrible wording (I mean, the wording is good artistically but again - rude up to the point of cruelty) that I had only one question - why Hawke is still alive? Ok, Merrill would never kill anyone (we do not count monsters here), but Fenris with his temper had at least to leave long ago or try to respond violently.
And with Izabella! At some point you literally call her a ****. And all those excruciatingly judgmental moral lectures like Hawke is the moral compass for the whole Thedas which Izabella - a very definition of free spirit - totally out of character takes with "thank you" and "oh, you are so right"... Same with Merrill, who dared to stand up to Keeper for her believes, leave her clan and her whole life behind for them, sacrificed everything for that believes and suddenly turned around for one man - the meanest SoB in Thedas!
I do see Hawke on the most rivalry part as a jerk, hypocrite, cruel monster, who is dancing on the believes of others just for fun and THEY TAKE IT AND CHANGE.
Yes, this is another problem - companion reaction. To stay on rivalry path companions believes have to be bended over and crashed. Completely out of character.
There are exactly 3 character where rivalry works - Anders (only because it's the main theme of the game), Carver (they made him this way that anything is rivalry with him and still, result of full rivalry if he is a templar is OOC for me) and Sebastian (guess, they learned by that point how to make system look more natural). For the rest of companions it's simply awful.
The idea could be good, but implementation did not work for me in.