Dear Mohan, As far as I can see none of this is of any real interest since it is just a quibble about classification. Cladists seem to suffer from Ralph Waldo Emerson's 'a certain foolish consistency is the hobgoblin...'. Since cross fertility, hybrid fertility (even self-fertility in hermaphrodites, which fish can be serially) and behavioural isolation mechanisms operate in quite different ways in various groups of plants and simple and complex animals there will by definition be no 'smallest group' species concept that is useful for all purposes. 

Jo