sorry to bump up an old thread, but i figured i would offer a bit of input from my experience with umbrella pine/italian stone pine...
so, my mom got one of those living christmas trees from home depot or something years and years ago...after the holidays she just tossed it in a big pot in pretty much full sun and did nothing to it except water it...after a short time, like others have stated, it began to develop adult foliage mixed in with the cute little juvi needles...the needles are quite long, and a deep green...waaaaay to long to be of any use in bonsai...so anyways, this thing just grew and grew for years with no pruning or anything...it actually grows pretty fast...i mean, considering it was left in a pot and given no fertilizer, no pruning, never transplanted and basically treated like an unwanted step child...it still somehow managed to put on about a foot a year and developed some really attractive bark on the trunk (not nearly as fast as jpb, but decent none the less)...like most other pines they are strong growers in the apex and the lower branches suffer for this and become spindly pathetic little things...
short story long, it got way to big for its pot and kept falling over...this is when she told me i could take it if i want it because she was tired of having to pick it up all the time...so i told her i would, not really ever expecting it to become much of anything...to fit it into my car i had to basically trunk chop it almost in half...this caused backbudding and reverted back to juvi foliage...
it is my opinion based on my limited experience that inducing juvi foliage by pruning is the only way this species can be trained as bonsai...styling is fairly easy though because the limbs are so easy to bend you can practically tie them in knots, and the tree seems to give you plenty of branches to work with...
as far as care goes...ive been basically treating it like i would a black pine, except that instead of "candling" like you would a black pine, i hard prune in fall/early winter taking more off the top and strong branches (kinda like you would with a spruce)...i have no idea if this is the best way though lol...i havnt read ernies article, so if anyone could shed some light on what is talked about there that would be super duper awesome.
hoped that helped a little...if not, oh well, it was fun to ramble about trees for a bit.
oh...almost forgot...keep an eye on spoke branches especially towards the apex...you can end up with an ugly bulge very quickly...this is true for all pines, but even more so with this species.