Lets see.
WD - Fsmash: I dont really use so i dunno when.
WD - Ftilt: When you're at the end of range for this (this is the longest attack you have). When your opponent is at a decent percent, or when they're off the edge.
WD - Utilt: I dont use this at all. D:
WD - Dsmash: Whenever you need to move a short distance and smash. (Mainly, when you CANT DC Dsmash cause the distance is too short.)
wD - Jab: Same concept as WD Dsmash, but with jabs.
WD - Grab: (I hope you mean WD - JC Grab), whenever you need a quick burst of speed before you grab. Gives yourself a lil' extra range. When runnin just isnt fast enough. D:
--edit--
Didnt see your 2nd question.
As far as I know, you can Ftilt everybody, but you want to make sure they're at a decent percent so that you actually knock them back some. If you do it at a low percent to, oh say marth - you'll hit him, but he'll still be in his danger range (tipper ranger), he'll just be like "BAM FSMASH *****!!" and well....... well. :/
You can also ftilt people for an edge guard, like if they're trying to grab the ledge from above, you can pop a ftilt out if you're not ready for a dsmash, or dont want to time a dsmash.