Serenians can handle other metals without issues. :3 It's just iron that causes a problem!
Honestly, how much they are affected by the iron weakness is entirely up to you, regardless of if they're full or part Serenian. It's kind of like, say, a peanut allergy! Some people can't TOUCH a peanut without getting sent to the hospital, and some people can eat peanuts and get away with feeling like they have a cut up mouth, but be otherwise fine. So it's up to you to decide how affected you want your character to be.