I once spent an hour researching fall physics and survivability. Strength of body, possible injuries. What the highest height a person has survived falling on record.
All for one goddamn scene.
As a comedy writer, I dont give a fuck about very realistic reactions to the stupid shit my MC does. But god forbid i let this giant slime monster fall faster than the MC because i didn't realize weight and wind resistance aren't a factor.
Yeah. I didnt understand that mass doesnt matter when in free fall, either.
Air resistance determines your terminal velocity, which is the point when you will not accelerate further while free falling. What matters is not only the mass but the cross-sectional area of the body that's falling. Let's say if a guy falls using the spread eagle position, he will fall slower than a guy going head or feet first, or in a diving position, this is because the air is affecting a bigger cross-sectional area, increasing the air resistance. The same happens with a parachute, it has an increased cross-sectional area, which increases the amount of air resistance applied to said parachute. both forces (gravity and air resistance) reach a stable point or terminal velocity, where you don't accelerate further and your speed is slowed down to a safe range.
Of course, it's not possible to use a rock parachute, as its mass, combined with gravity, will stop the air resistance from slowing it down. The material used for a parachute has to be light while also being resistant as to not be torn apart by the winds, and what matters the most is the size of it.
So yes, a giant slime monster, unless it stretched itself to form a parachute, would fall at the same speed as your MC. (This made me think about Elastigirl from The Incredibles when she transformed herself into a parachute to save her baby.)
Also, about the main topic. I would say that it all depends on each one, but what I consider is the most important rule is: Keep to your world's rules.
Consistency depends on playing by the rules you set for your world. For example, if you said that ANYONE in your world could fly, but someday you decided your MC or a villain couldn't flight because it was convenient to the plot, that's breaking your world's rules, and that generates consistencies problems on the plot.
UNLESS! You give it a convincing explanation within your world's rules, like for example, everyone could fly because they got wings, but your MC or "X" Villain lost their wings, or they were trapped with some mechanism that didn't allow them to flap their wings and fly away or something like that.
Fantasy, is that, fantasy. You don't have to follow real-world logic, you can use it as a guide to building your world, but at the end of the day, you as the writer and creator of that world have the last word. What's important is to keep consistency within your world rules and logic.
