how to enable different number of revisions for different blogs when using WPMU?

No, wp-config.php can have granular constants. Like this:

if ( 'example.com' == $_SERVER['HTTP_HOST'] ) 
{
    define( 'WP_POST_REVISIONS', 1 );
} 
elseif ( 'blog1.example.com' == $_SERVER['HTTP_HOST'] ) 
{
    define( 'WP_POST_REVISIONS', 5 );
}
elseif ( 'blog2.example.com' == $_SERVER['HTTP_HOST'] ) 
{
    define( 'WP_POST_REVISIONS', 10 );
}