It was comfirmed by Nomura that the other members had weak heartless. Only Xehanort was able to keep a strong heartless. Look below.
Q. When was the XIII Organization founded?
A. The timing is when Philosopher Ansem's 6 disciples turned into Heartless. The XIII Organization is called like that just because it has recruited 13 members, but it doesn't matter if it has "XIII" in its name or not. All 13 members were already gathered in KH:COM but the members that were in Castle Oblivion still called it as the "Organization."
Q. Besides Xehanort, did the other 5 disciples that turned into Heartless disappeared?
A. Not really disappear but more like they're somewhere else as normal Heartless. Xehanort was about the only one who was able to keep a human form.