Yup. Every named knight can be recruited on the scene you find them in (unless you don't have their item, of course), and if you miss that chance you miss that particular knight until your next playthrough. Key items are like, always found in objects.
And only Maeria, Lance Knights, and Duelists can move. Their Chaos attack moves them, and it moves the entire "spawn point," so if those particular units won't be able to hit an object, you can still still move them to a place where a different knight will be able to hit whatever you're trying to hit.
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