The encounter data requires "GAME_CLEAR" for all of them except the four I listed, but it has no flag at all for these, while others like Orthworm are clear about requiring some other specific flag and I assume the same would apply here if these four actually required beating a specific Titan.
I know that Great Tusk and Iron Treads can be found as one-offs after the Titan battle, but the encounter data does not obviously support that any of the others have anything to do with that.
I would like to leave the question open to someone who has actually tried it, if that's okay! Or maybe I'll just try it myself tomorrow, but I won't be able to start playing for a solid 24 hours - if someone else wants answers before then, it's definitely worth giving an actual shot and not just making a conjecture!