i consider midrange (for ganon, at least) to be from the start of his foot to the end of the disjoint on his f/dtilts, or, if aerial, edge of nair/fair to the start of the foot/hand. far enough away that he can avoid most normal grabs just on range alone, but can also still buffer roll/dodge to get out from marth/ddd/dk/zard length grabs and tethers safely.
ideally, downb never gets used unless you're tech chasing and want to get a kill or a setup for one as it's unsafe on whiff and shield. however, everyone has certain habits that you can use to their disadvantage and try to bait them into doing it. the safest place to bait it is just right at the very edge of its max range (just go into training and get used to it) so you can literally just react by moving back then punishing. as for habits, that's a player by player thing, you just need to learn how to manipulate the people you want into doing it.
flamechoke is much the same as wizkick, but it's range is lower. it shouldn't be used blind, but really just as part of a conditioning tool for the ganon. it can be a bit tricky since there are technically 3 different lengths it can go (normal grounded, b-reverse grounded, and aerial). b-reverse grounded is slightly longer than normal grounded, and aerial is the shortest. b-reverse and normal grounded are the same startup and end lag, while aerial has longer startup (if you're jumping first, otherwise it's the same) and longer endlag (due to falling to the ground and the ending animation actually being a bit longer).
there's no real "always pick this move" situation for either move. they shouldn't be used unless you have a strong read (wizkick) or have been conditioning the opponent into shielding a lot (flamechoke). usual conditioning setups for flamechoke, though, tend to be getting you on a platform and pestering with uair/fair from horizontal/underneath, then switching into flamechoke at close range, or tech reads (so don't be predictable with them!). otherwise, you can find people fishing for it if you're near the ledge, to go for a ganoncide or a slamjam, or just to try and build more damage out of a combo that would've just reset to neutral. you can beat fishing by just calling them on it and playing smart. out of a combo, though, not really, that's how combos work.