That... Doesn't make any sense? Like, if wanting to be happy makes you a slytherin than the house would be a lot fuller. A slytherin would have found ways to be happy in the first order by gaining power, which according to the novelization atleast, he was on his way to doing since he wa good at what he did. He didn't come across as a master strategist using Poe so much as someone latching onto the only opportunity they had to escape a situation where he'd be forced to act against his morals. Also rushing onto a planet to save someone when you don't have any real plan is the most gryffindor thing in the galaxy.

I mean yeah, he was tempted to leave to the outer rims, but that's a necessary moment to show that he has other options, that he was tempted to run and save himself, but that he chose to help others instead. I don't know how you can claim his actions were based primarily on self interest/ambition. He went in knowing he might die on Starkiller but did it anyways