But if you're referring to a set of birds and fish together, then the usage is "all y'all" because the canonical partition yields more than one subset (one containing birds, and one containing fish). The distinction helps differentiate between whether you mean the superset or one of those subsets.
It works with any other partition which might be obvious (not just birds and fish). If you have two families together, you might avoid "see y'all later" because it could be interpreted that you only expect to see one family later. "see all y'all later", by contrast is unabiguous--you mean both families.
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Did I get it right? Am I a true Texan?