@Scyphi
Likewise, I don't think they were meant to be any specific character in the story, the holoprogram just considered them an extra supporting "player" and had the program's characters assume they were miscellaneous acquaintances/friends of Dixon's he happened to be with at the moment and otherwise treating them as, at most, the equivalent to a "hired goon" for Dixon if they proceeded to actively assist him in solving the case.
Basically, to the holoprogram, they were effectively like redshirts who weren't super relevant to the story overall, and just there to fill, again to the holoprogram, an otherwise minor role. If they ever did anything that forced the holoprogram to treat them as something more relevant than that, then it'd adapt accordingly, but they'd still almost certainly always be treated as "second-fiddle" to the Dixon character, simply because that's what the story demanded. Dixon, after all, is really the sole intended character of focus in The Big Goodbye holoprogram.
As for the identity of their characters, I'm assuming the holoprogram would just use whatever name the person playing Dixon happens to introduce them as, or whatever they themselves happen to introduce themselves as if Dixon doesn't answer for them, so it'd really be up to the players to determine. There's no preexisting character they'd be assigned as, at least not by default--the story isn't geared with that in mind.
So does that also mean that Carlos (the role that Data played) was not a specific character in the story too but was made up by Data to cover himself, or are Carlos and Gloria (the role that Guinan played) actually characters from the Dixon Hill story? Just curious.