Personally, I feel that his past as a cannibal mercenary shouldn't have much bearing on how he gets in. Unless he was such a notorious douchebag that they've heard of him even in Thaln, or his jungle homeland is right next to Thaln (which would be a bit weird), they couldn't have possibly heard of him. Considering Themerlinhawk's character kind of walked down from the Velt foothills and petitioned to join, it seems like the only real requirements are being a capable fighter and/or being prepared to do good deeds.

That said, it still begs the question why a guy from some far off land decided to root up and join a distant knight order. If he just wanted to do good deeds, the Iron Rose knights can't possibly be the only people that help people in this whole wide world. I think people just think there should be a more specific reason for choosing to join the order than random whimsy.