It wouldn't mess, not by default. It would change the team dynamic - but so does characters developing. A Tahu that is angry changes the dynamic when he grows and learns how to control his anger and impulsiveness.
They are warriors, but that's not incompatible with romance. Nor does romance always necessarily "get in the way" - it can, but only when it's done badly.
It doesn't need to be.
"Just because it's never been done before" isn't a reason not to do something.
All romance is is an evolution of two character's relationship. It does add a new layer to the interaction, it's a deeper relationship, it's a new relationship, but not anything different. Pohatu and Kopaka becoming best friends changes the group dynamic.
What you're trying to say is badly written romance. Badly written romance is pointless, and just wouldn't add anything to the team dynamic or make the team's plight more engaging.
That's the issue I keep seeing: most people assume that all romance is going to be written badly, and all arguments are only from that implication. I don't agree with that.