Only if the PC is part of a set of some kind. Otherwise, it's just a PC.
And, by "set", I mean a collection of PCs that was produced in some sort of quantity, even if only two copies of each PC were printed. Otherwise, it's just a PC.
A single known PC of some Negro League HOFer shouldn't be considered a RC or card of any sort simply because there's so little else available on that player. It's just a PC.
Same rules apply to cabinet photos of 19th century players, even if more than one copy is known. They may be lovely, but if they're not part of any series, they're
photos.
I think a lot of the rule-bending, for lack of a better term, can be placed on auction houses trying to squeeze more money out of buyers by convincing (some of) them that items like these can also be considered cards. Clearly, many people have taken the bait. Again, it doesn't make these items any less fetching in most cases, but let's call a spade a spade.
I suppose that someone will counter my feelings with pointing out that the Peck & Snyder is considered the first baseball card, yet is not part of a larger set!
Due to the advertising on that card, I'd like to think of it more as a premium, but there are technicalities everywhere we turn! Hey, we can call it a "set" due to the variations!
These aren't hard and fast rules, but they're my take on it. The best part is that the only rules are those that you have chosen to consider in your own collecting. If you consider that elephant over there to be a card, so be it! More power to you.