Oh, his beliefs are very slippery, and as you say, sometimes incompatible and even contradictory, but within a particular dialogue his actions/theories are based on particular on principles or beliefs he arrives at through discussion. Within the context of a given dialogue, Socrates usually starts off probing whether or not a given belief is true or untrue, and having established that it is true (in those instances where it is), applies the principle to the subject under discussion.
Now, Plato's dialogues are often inconsistent, and you are right in saying that he often uses theoretical individuals as the basis for his attacks. However, in my opinion Plato (and by extension, Socrates) appears to believe that given the right principles and beliefs, men will in general behave and act in the right way (see The Republic). For this reason I feel that Plato does not so much criticise 'the many' as criticise the beliefs of the many.
Of course, there is an implied criticism within that, in the sense that Plato never lays the blame for their beliefs on anyone else, so that they themselves must be to blame for believing what they do. Nevertheless, he clearly thinks that given the right principles, 'the many' need not be so foolish. Though they are in fact ruled by the Guardians in The Republic, he clearly feels that 'the many' will naturally see the advantages of such rule and accept it, provided their principles are sound.