Many of these unaccounted for characters were actually associated with the jedi at one time or another.
Assaj Ventress (the black and white lady with the hissing voice) for example.
General Grievous didn't actually use the force, and could never be considered a candidate for knighthood as a jedi or sith lord. He was a cyborg of sorts, engineered specifically to help reduce the numbers of jedi knights during the clone wars. He proved himself to be too much for the weaker jedi to handle, and a serious problem for the stronger jedi, and was known to take on several at a time, beating them at their own game.
The Emperor's Hands could have easily been found by the jedi order, if there had been a jedi order around to find them at the time. Their place in the galaxy came about after the jedi purge was initiated.
I must admit, I've never heard of this particular Grand Admiral before. Who was it?
Darth Vader's apprentice exists for the sole purpose of selling a video game. Though Vader's taking of an is a possibility, perhaps it should be a road left unexplored. Vader wasn't the type to take an apprentice. His son is the only candidate he would ever consider for that role. Vader seemed a very hands on character, and would rather do thing himself than send others to do them, unlike the Emperor.
The people who have created all these characters come up with pretty plausible reasons for having them, though most of the time these characters aren't very good ideas. Still, it's supposed to be a rather large galaxy. If these are a thousand people out there who can use the force and haven't been found that is still a tiny number, and would be, even in a population of only six billion