I'm kinda biased. I'm that dude that came in as an E1, and clawed my way to the top, so I think EVERYONE should come in as an PVT. It's good for the soul to have to start from Square 1...
But to unbiasly answer you questions... Yes, promotions from PVT-SPC are "automatic"... per say. If you're an outstanding soldier, or if you at least have good NCOs, then you can get promoted fast. If not, then you'll have to wait for it to come to you.
Private (E-2) - Six months time-in-grade (TIG) as a private (E-1).
Private First Class (E-3) - Four months TIG as a Private (E-2) and 12 months time-in-service (TIS).
Specialist/Corporal (E-4) - 6 months TIG with 24 months TIS.
You can get waivers for each rank that can get you promoted a couple of months quicker. That's where being highspeed comes into play.
As for SGT, thats 8 months TIG as a SPC/CPL and 36 months TIS.
Is it easy? Nothing's easy. But it wasn't hard, if you know what I mean.
PLDC is now called Warrior Leaders Course (WLC). And it's so easy it's ridiculous. That doesn't mean you should take it lightly though.
Don't worry about the SGM's Academy. I'm sure it's terribly difficult.
As for "Stripes for Skills"? Isn't that a Reserve program? I don't know anything about that. Sorry.