Yeah, as a cheesey rules exploiter. If I were to even countenance letting a character keep a sacrifice creature around to keep a hex going, I'm going to require something more substantial rather rhan a totally insignificant worm or squirrel.
Yeah, I'm not sure I'd call it rule-exploiting. At least not with any real negative connotation. Sure, such finagling can seem trite or cheesy or manipulative, but it can also be indicative of good role-playing.
Whatever character you're playing exists in some world, a world of the 5th edition designers and your DM's making. It's imperative on players to understand that world and the laws that govern it. I wouldn't penalize or berate a player who was trying, however creatively or exhaustively, to understand that world. I think it makes the game all the richer. If a player wants to understand the logic behind magic and specific spells, I don't think there's anything wrong with that.
These inquiries can, of course, put a lot of stress on the DM to actually flesh out and understand and intelligibly respond to questions about her or his world, but that's what being a DM is all about. (It's also about balancing the fun and involvement of all the players at the table, and while sure, a particularly inquisitive or self-absorbed player could take up too much of the DM's time and energy on such finely delineated concerns, I don't think such concerns are in and of themselves bad, so long as they're respectfully vetted and weighed within concern for the fun of everyone involved.)
All that said, the rules surrounding the Hex and Hunter's Mark spells seem poorly framed---allow the spell to be transferred upon the target's death (allowing for whatever size "target" deemed appropriate), or allow the spell's magic to be maintained throughout its duration without a target, or disallow the transferring of the spell beyond one target in the first place. It seems to me a bit hokey to both allow transference and yet to say that it can't be transferred to anything you can carry "because that breaks the rules."