I haven't see all Templar PT video from Mel, so a question: in general, is there much bickering/quarrell with Cullen due to "Templar" things? Or is it more bitter things left unsaid? How is he reacting in general? I'm starting to think that I might want to try that so maybe I won't see Mel's video but still like to be spoiled 
I haven't found any bitterness in the romance itself. They say everything that needs to be said, but none of it is bitter. There's that initital disagreement on whether to become a Templar, but Cullen ends up being supportive of the IQ either way, and his lines when you tell him you did it anyway are still very sweet. Otherwise, he's as loving and respectful as with any other class or spec, other than the mage (where he's constantly apologizing for himself instead, but that's for another conversation). I used the word "challenge" there, but I mean it in a positive way. They challenge each other's lazy assumptions, but not in a bickering way - in ways that are respectful, supportive and constructive for both of them.
If there is any aspect of bitterness to my own playthrough, it's in the decision to keep him on the lyrium, but that's independent of the IQ's class. You can romance him as a Templar and still choose to support his sobering if you don't want that added weight to your story, and you can still keep him on lyrium without being a Templar if you want the angst, so it's an added side feature rather than something the Templar romance itself brings in. Still, it was the right decision for this particular IQ at the time, and the fact that she keeps her word to him to the end made me even more certain in it (as in, the first line you can tell Cullen immediately after defeating Cory is that you made him a promise and you can now start looking for safer ways for him to quit the dust -or both of you, if you're a Templar too, which rounds up your Templar romance perfectly in the end).