His legs are intangible on start-up for a bit, but once the hitbox is out the intangibility goes away. Its like frame 2 or 3 until frame 7.
I am never sure when its a good time to pull off a Dsmash on anyone. So many things in this game are "I could use this option and it would be really good if it works. If it fails though, I get hurt pretty bad." I think this is pretty important in melee. As a marth, you could try going for easy gimmicks like Fthrow -> fsmash on a fox hoping they would not tech, but something like won't work on against a good fox. Should I rely on mix-ups to get me through, or should I rely on the guaranteed stuff. Marth can guarantee a 0-death on fox, hard, but its guaranteed if you don't mess up.
In brawl, I am wondering the same thing. Should I try mixing in dsmash and gatling into my jab cancels as mix-ups? The reward is good, but if it messes up I'm getting punished pretty bad. If I go with shield or ftilt after a jab cancel, the reward is less, but is a lot safer and more likely to work. Or with Dthrow at higher percents, should I go with the hard read of getting a bdacus or go for the dash forward to be able to react to either DI option?