The amount of hitstun a character takes is proportional to the initial KB velocity (not total KB distance, the initial velocity) – this multiplier is a constant, and does not change between characters or attacks at all. If you are sent off with a high degree of KB you will have a high amount of stun. As an aside, meteor attacks that bounce off the ground still get the full amount of stun as if the character were being knocked downwards, but the distance is cut short by the bounce – this is why you get so much hitstun advantage out of meteor bounces.
Anyways, most 50-90° combo moves rely on gravity to pull the opponent back down after being hit to some extent. Compared to being hit horizontally, gravity pulling on a vertical or partially vertical launch reduces the distance the opponent is sent away from you overall *without* decreasing the initial KB velocity... if the opponent doesn't travel as far but has the same stun, you get better followups. Opponents that fall faster will get pulled down by gravity more so they stay closer to the attacker, which nets more of a followup advantage. Floatier characters aren't pulled down by gravity as much, so they they usually get launched a bit further vertically without as much gravity to help pull them back down and give you good followup advantage. So yes, lower fall speed increases combo resistance.
On the flip side, higher fall speed also means that it's easier to stay grounded or become grounded and tech attacks. This adds its own sort of combo resistance, trading combo followups for tech chasing followups. Depending upon the attacker and the stats of the opponent's tech rolls, this might be easier or harder than comboing the character directly.
The most comboable characters, link, roy, lucas, diddy, etc, are all characters that fall just fast enough to give the attacker good hitstun advantage off vertical(ish) hits but don't fall fast enough to frequently reach the ground and force a tech chase.
Weight is a factor in how far you get sent, regardless of the direction. Lighter characters take higher initial KB velocity, while heavier characters get sent shorter distances due to their lower initial KB velocity. It's important to remember that in both cases you receive hitstun proportional to your initial KB velocity. Even though a light character might take higher KB and end up getting sent further it'll still receive more hitstun to match. This is why lighter characters aren't inherently harder or easier to combo, per se.
Weight *does* affect throw speed, though. Lighter characters usually get thrown faster, so the attacker has less cooldown and can follow the opponet sooner for more advantageous followups. As an example, spacies get chain grabbed on u-throws so badly because they're light, which means the attacker has low cooldown for quicker regrabs, and the spacies have high fall speed, which means they get pulled back down a lot so they don't get sent as far from vertical throws. So even though low weight doesn't directly affect combo resistance, it can sometimes make the character more vulnerable to throw followups.
The interactions between character size, fall speed (gravity & terminal velocity), and weight are what create the diverse range of followups and combo % ranges in smash.