One of the unique aspects of Melee is that move knockback changes depending on damage. Different damage requires a different approach to the punish game.
Chainthrows against Fox are guaranteed from zero to 30-something, and against Falco from 3 or 4% to 30-something. If you miss chainthrows in these percents, it's simply that you're failing to perform the inputs. If you're sure you're performing them correctly (grab or turnaround grab until ~17% then dash->pivot JC grab against no DI the rest of the way) then all you can do is practice. After those percents, there are ways for Fox and Falco to get out if you just keep uthrowing them, so, in fact, against experience players it should really stop working right when you say it starts working.
With regard to other combos, you'll need to ask more specifically about options against a given character, because characters have different weights and fall speeds which change what you can do to them.