Probably, the best things to do, if you need to stay on one single wiki, is to work at namespace level settings the appropriate permissions on each group of users.

As example, you can have two namespace called PUBLIC and SECRET.

The content of the first namespace is available to all the people, while the second is available only to a specified group of users. Anyway, as other people stated, you must be carefull when writing your page, because a link to SECRET namespace page from a page of PUBLIC namespace could be a pithfall in your "strategy".

G.