Someone correct me if I'm wrong, but there only appears to be a noticeable difference between a "Smash" Forward-B and a tilt Forward-B (as in, how you get Samus's Power Missiles and Homing Missiles, respectively) for Squirtle when you start it in the air.
That said, a Smash Forward-B somewhat close to the ground looks like it would be a useful mix-up between low aerials. Withdraw gets slower the longer it's in the air, but done directly from a short-hop, it's -slightly- faster than Charizard's run, so if we get opponents to shield, we can hit their shield and move away before they can punish us. If they shield it, and they fast enough to keep up with us, they won't be able to grab us, because dash grabs are slow and/or have bad range, so worst-case scenario, we stop Withdraw immediately off the stage with a midair jump and Up-B intact.
Also, I just found out that we can stop Withdraw on the first possible frame by holding Back. Doesn't require good timing, buffering, or anything. Just hold the direction opposite of where you're moving, and you'll get out on frame 60 of the animation. Cool beans.