Does it depend on the gun like a rifle load does or is there a bit of variation in loads a gun will shoot? Is it all depending on the intended use?
Yes.
All of the above.
I have to 20 gauge flintlock guns. Preferred loads are similar, but not exactly the same.
Small game, roughly, 60 to 65 grains 3f or 2f, thin card, lubed wad (either felt or balled up lubed tow), about 1 oz. of shot, held in by another thin card. Small game loads aren't that picky.
Turkeys. One gun does fine with 80 grains powder the other doesn't do well with more than 70 grains. 1 1/8 oz. #5 shot. Skychief load on top of that for one gun, haven't tried it in the other yet, was using a lubed paper shot cup.
Roundball for deer. One gun likes 85 grains 3f and a tight patch/ball combo, the other shoots wild with any tight combo, but will shoot a loose patch ball combo (.595 ball and .010 patch) with either 65 grains 3f or 80 grains 3f, equally well. That second gun also shoots a .610 ball over a lubed wad and held with a card over 65 grains 3f, the 1st gun wants about 70 (experiments are ongoing).
Loads must be developed either way. But I do think there is a little more forgiveness than with a rifle.