Wow, never knew about the Zero switch...that's interesting.
Anyways,
KO moves- His Usmash will KO at very low percentages. Probably a bit below 100 with good DI. I'll have to check on that.
I tested this in training mode. I made ZSS stand at the center of FD (made her die and respawn, then didn't move her, assuming this was the center) for a constant position, and I put Squirtle right next to her, and used usmash with him. As ZSS, I was able to live to 144% with DI. At 145% I died, no matter what I did. But I could live pretty easily to 143%, and though it was difficult and needed like, godly DI, 144% was possible as well. I was able to get 144% consistently, it's just tough.
Note - this was with uair momentum cancel only. I did not use flipjump, nor regular jump, to aid breaking my momentum. And... I don't really feel like testing them. lol
EDIT: I dunno if Squirtle has multiple hitboxes on this move, or if different 'spots' have different knockback and whatnot. When I tested this, Squirtle's attack would do approximately 11% and ZSS would be in the middle of the "stream."
His Dthrow is his best KO move and should KO @ about 115 off the edge of a stage.
Tested this on FD in training mode as well. I made ZSS teeter at the edge, then made Squirtle dash grab her so he was literally on the edge and she was offstage. I did a dthrow.
This is what I got:
Uair momentum cancel only - 162%. At 163%, she died, no matter what I tried.
Uair + flipjump - 167%. at 168%, she died, no matter what I tried.
There's 2 ways to DI this dthrow Squirtle has. One is the traditional way, up and towards the stage. If you do that, don't use the flip jump, you'll kill yourself early off the top. That's how I got the first result.
The second way, using the flip jump, is to hold down and towards the stage. Basically, aim at Squirtle's feet or something. Use uair to momentum cancel and the flip jump just after, and you'll get the results I got for the second method.
Again, you gotta DI like a god, but at least you know it's possible.