Let's see how much I can remember about glider flying in the US.
First, no medical required, you just have to self certify that you're ok - which is why a lot of ...old people do it
Second, you need a license to fly. You can get private, commercial instructor.
I believe there is an instrument add on rating that's available, but I don't know anyting about it and I think it's pretty rare.
You can fly a glider VFR in class A (as I believe you can any aircraft) though you need a special clearence and they block off a section of the airspace for you (assuming they say yes of course).
As for actually flying in the clouds, you probably dont' get too much as the sunnier the day, the more thermals you get - at least in Texas
I don't know anything about orgraphic lifting
Thermals top out in clouds so you can thermal up to the bases and then sort of get a push around and over the top - works in a 182 as well btw.