In a 90g, the 6-line probably won't give you too much trouble, but that's not a given.
One thing to keep in mind is that a melaranus might eat the flatworms. I've seen too many tanks where they didn't touch them. If there is a fish that you want, and it ends up eating flatworms, that's great. But don't add a fish you don't want just because you think it will eat them; you risk being stuck with a fish you don't want that's not doing what it was supposed to do.