I know I'm kinda late to the party, but I do this on purpose with Wolf a lot and have had it happen quite a few times with Squirtle (on Lylat as well as SV, FD, and BF iirc). For Wolf, it happens at two main spots. I THINK this is because Wolf's traveling animation during his sideB takes up two frames, meaning that he'll actually "exist" at two different spots throughout the sideB. Only when you're at one of these spots (hence, only when you're at one of two specific distances/angles) are you able to pull this off.
I don't know exactly how it works with Squirtle, but Wolves in general tell people to aim a bit above the ledge if they want to "lightstep". The angle of Wolf's sideB is probably something around 30 degrees, so maybe try aiming to hit a bit abovethe ledge at that angle? There's probably a certain part where, if you start your upB from there, you should get consistent results.
If you buffer a jump after hitting the ledge (generally the best idea), you can either attempt to land on a platform or get back to center stage from above (Wolf can shine away opponents from below, Squirtle can use dair because it's amazing) or jump backwards to edgeguard opponents with a bair. You'll still have the extra recovery lag when you land onstage, so keep that in mind. There's no invincibility either, all you do is get your second jump back... so just make sure you don't get read or you could be in for some trouble :x Squirtle can probably fair right away and still recover to the ledge though, or maybe even land onstage because of how he pulls up his legs, which would be AMAZING.
Basically, look into this for Squirtle, at the least it'll be a useful trick to pull out every once-in-a-while