True, Jacen only really learned from Lumiya, who like Ventress wasn't truly Sith (TCW's not the only one that played fast & loose with the Rule; the EU in general did, too, like the REPUBLIC comics featuring many acolytes that weren't ever actually identified as Sith: Ventress, Sora Bulq, Tol Skorr, probably some others I'm forgetting; they aspired to be Sith, but were only ever tools of Dooku and Sidious; of course, Darth Maul was the same according to the Plagueis book, but be believed himself to be Sith and got the 'Darth' title too)
Acolytes were not considered sith, just 'assistants'. They were given minor/moderate training in the dark side to hook them in and make them useful, but they'd never progress past that point into a sith lord. There was only ever two 'true' sith during Palpatine's reign: Palpatine and Vader. Everyone else was a throw-away tool.
Darth Maul, while he may of had the Darth title, was little more than a placeholder for a better apprentice.
Why? Krayt learned to be Sith from an ancient Sith Lord... and Vergere... he found many old holocrons and consulted them regularly. Some, like Bane's holo, considered him an imposter. Then there's the Lost Tribe, who may or may not survive FOTJ.
Minor FOTJ spoiler:
Krayt broke the Rule of Two and instituted something that was like a mix of Kaan's Brotherhood of Darkness and the old Rule by the Strongest (both of which failed horribly). That is why Bane thinks he is an imposter and not a true heir of the Sith. If I remember right, the ghosts/projections from pretty much all the sith holocrons he had mocked him for being weak. Which kind of invalidates his reign if he can't even get a little grudging respect out of them.
As for Vergere, I'm not sure you can really classify what she is. Nominally sith, sometimes jedi, other times who knows?