ie can certain pages be locked down once they have been created?
You can lock pages to prevent edits (only Admins/SysOps can edit).
You cant lock single pages from viewing by other users (who can view the rest of the wiki).
And, can I create something with mediawiki that has the categorisation of the Twiki system?
Perhaps you should explain a little bit more detailed, what u are looking for, because not many users here know the twiki really good.
Sincerly, Jan