I think Charizard does not actually have any Super Armor on his Side-B. If Rock Smash has SA from frames 1 through 19, Charizard being interrupted and uselessly dropping the rock would be nigh unheard of: Charizard would fumble the boulder only when hit on exactly frame 20, i.e. during the 1-frame window between the end of the SA frames listed (19) and the first frame of the attack (21, which is when he shatters the rock himself). Since I see the rock fall uselessly moderately often -- at least, more often than single-frame precision would suggest -- I decided to do some tests.
I Rock Smashed facing away from a laser-shooting Falco on Final Destination; every single laser hit Charizard, dealt damage to Charizard, and flinched Charizard. Each time the laser hit him before the rock was smashed, it dropped and disappeared. Each time the laser hit him after the rock was smashed, the rock finished shattering and Charizard suffered the minimal knockback of Falco's lasers. When I tried Side-B'ing towards Falco, some of the laser shots hit and shattered the boulder; when this happened, Charizard received no damage or knockback because the lasers stopped on the boulder and never reached the dragon himself. The lasers that missed the rock had results identical to when Charizard was facing away from Falco.
Figuring there could be some strange rules like "boulder shattering early = SA frames", I tested against ROB this time, as his laser passes through things. Now Charizard was damaged and flinched every time, regardless of his facing, regardless of whether the eye-laser shattered the boulder or not.
I don't have the set-up to actually test this frame-by-frame, so I could be missing some Super Armor frames that are actually present in the move. You list yourself ("Charizard, contributed by NoVa") for that section of your post, so I'm wondering how you got the results you did. What attacks did you try on Charizard? Do you have some video of Charizard taking damage while using Side-B but finishing the attack anyway, without flinching?
/RtEB