Ah, okay, they did essentially crew the Titans, thanks! Interesting, that, and does nominally leave room for other types of Masters. Just have them also be crew in-story and happen to lend power in different ways
(But yeah, better to have "Masterforce" as a cluster with subtypes, rather than them all being Titan Masters), ; if nothing else,
Tlthe toy-lore matching might be a bit tricky because then you'd have, say, 3 engine bots and 3 bulks who aren't Headmasters but conveniently do have cavities for engines.