Yeah, that's the exact problem with mixing posts. Wood soaks up water and swells, then dries and shrinks, but metal doesn't move. They'll never settle the same way. You can try to brace it with a really solid top rail or extra concrete, but honestly, it's a band-aid. You're always going to be fighting that uneven movement, especially after weather. I'd bite the bullet and replace the wood ones with metal to match, or you'll just be straightening that line every season.