I see the 'Flash' move having the pokemon out walking around with you, and it's probably better than having a fire pokemon out to an extent, though I've used the 'Char' option in Mt Moon in my fic, but there's more dangers to a Char, such as bad air, possible explodeable items or coal, or what not, and yeah, just to kinda make it realistic... or I could have gotten Mines confused with caves...
Anyways.
Yes you probably would get lost in the cave, as most caves are vast, dangerous and to not be traveled alone in, as though they look safe, they aren't. So it'd probably be a true test of the trainer's will and bravado. And probably how stupid they can be when they want to quickly get from point A to point B.
And pkmn wise I see it like this;
Pokemon like; Magmar, Lanturn, Charmander{meleon/zard} might have a radius of five to fifteen feet, maybe twenty, around themselves, or rather the light source they could provide. This could be cut in half as the light source wouldn't be able to be cast directly infront or behind on some of them, and that if you were right by them, it would also cut off the light source previded a bit, in whatever direction you're standing in.
Pokemon like; Umbreon, Pikachu would have a different way of using Flash, but as the show showed, Umbreon wouldn't be able to battle while using it, and while bright, and probably more effective than the previous pokemon, it tired the Umbreon out. I also would think Pikachu and other electric types like Pikachu would be affected the same way, being unable to battle and quickly able to be tired out.
Pokemon like; Girafarig, Drowzee, Abra, Espeon Psychic types might be the best to go with for Flash, as I see the more experienced the psychic, the better they can use the move Flash, and instead of making their bodies glow, they create a little ball of glowing psychic energy, that doesn't do any damage, but does give off a bright light. And the bubble probably might pop/burst into a bright flash if the psychic pokemon gets distracted or forced into a battle.
And back on the cave part, I think it'd be best if you studied about caves a little, learned what makes them tick, wether you want Dark cave to be 'dead' or 'alive' (dead = dry cave, alive = damp cave).