It is widely known that Plato seems to be committed in a number of dialogues to the view that all perfections are «united» -- whether such unity is construed as identity, which doesn't lack textual evidence, or, more probably, as some kind of mutual «supervenience». (See for instance Laches 199e3-4, Alcib. I 114d-116d, Protag. 329c-333d & 349a-c. Whatever the solution to those interpretive problems is, what anyway can be ascertained is that, when writing the Statesman, our philosopher is keen on maintaining that not only is it not the case that all perfections are identical, but, moreover, some perfections do in fact clash with others, which means that a thing can possess one of them only to the extent it lacks the opposite perfection. However, as we're going to see straight away, the Statesman's main purpose and thrust is likely to be that of emphasizing the necessity of some unity among opposite qualities.
The significance of such a contention can be set off against what will become the Aristotelian (and in effect the commonly received) view on the topic. In the Statesman Plato recognizes that in each case there is some desirable mean between the extremes, but where it lies changes according to circumstances. Trying to secure that convenient mean doesn't debar us from loking upon the extremes under consideration as virtues or perfections themselves.
Thus, when the Statesman is drawing towards its conclusion, the Foreigner abruptly brings up the issue of ὑπερβολὴν καὶ τὴν ἔλλειψιν (283c3-4), and thereby that of the art of measuring (ἡ μετρητικη). Immediately the problem arises of the relations between the opposite extremes. In Theaet. 152d, 157a, 160b-c, it appears that Plato distinguishes relative from non-relative properties or determinations in the same way as he does between empirical, earthly, changeable things and the Forms. Yet in his later dialogues Plato is clearly committed to holding the view that the Forms themselves enter a number of relations, and even that some Forms partake of others.
Since very early onwards he seems to have been concerned about some reciprocal relativity of opposite qualities (there are many places where such relativity is stressed, as Charm. 168b5-169a5, Phaed. 102c, Hipp. Mai. 288e1-281d11, Resp. IV 438b-c. Thus in those dialogues Plato seems to have regarded mutually opposite properties as related mainly (if not only) to one another and to nothing else (notwithstanding the difficulty stemming from his strong leaning towards a conception of Forms as utterly non-relative). Now (283d6ff) an important distinction is made between such mutual relatedness and a quite different one, in virtue of which each of the opposite qualities is related to τὴν τοῦ μετρίου φύσιν (383e3) -- equivalently τὸ μέτριον (283e11) and (284c1) τὴν τοῦ μετρίου γένεσιν, and probably τἀκριβές (284d2). Such relatedness is in fact much more important. Plato compares such finding with his having compelled Nonbeing to be (284b7). Inbetweenness among the extremes thus characterizes whatever the arts deal with (284e6-8).
So Plato seems very close to [what we are entitled to call] the Aristotelian view. But such appearance vanishes very soon, when (starting on 306a9ff) the Foreigner argues for the thesis that prudence and manliness, while being both parts of virtue -- and so virtues themselves --, engage nonetheless in a fierce mutual enmity, which confessedly is an amazing claim (306b13-c1).
In the same way as in Sophist and in Parm. Plato argues for the mutual community and interconnectedness of Forms (even of opposite Forms which thus emerge as not entirely contrary to one another), now he proves that manliness and prudence -- as well as many related qualities -- can be mixed or mingled (or combined, or joined together: 308c6, 308e7, 308e9, 309b2, 309b7, 311a6; the words used are: σύμμειξις, συνδεῖν, συμπλέκειν, κοινωνεῖν, σύγκρασις, and also [310a4-5] a very divine tie [θειότερον δέσμον] among the parts of virtue which are discordant by nature and drift towards opposite extremes: 310a4-5).
By the way, an apparent difficulty can be easily dispelled: isn't Plato on 307c4-5 contending that opposite virtues do not consort with one another, that they are not to be found together in action? Yes, but only σχεδὸν ὡς τὸ πολὺ: 307c2: almost always, or in the main.
Plainly Plato is here availing himself, at least in part, of the same words he also uses in other late dialogues in order to convey his new approach to Forms, as enjoying mutual participation and community, even when they happen to be contrary among themselves (contrariness turns out to be only real up to a point, never absolute). The final lesson of the Statesman is thus a confirmation of Plato's late doctrine of the unity of opposites and some compatibility of sorts between being-so and not-being-so, in virtue of the existence of degrees. (Which of course doesn't entail total rejection of the principle of noncontradiction, but rather a qualification thereof.) The statesman's art is wisdom to find the suitable degree in each case, to accomplish the just alloy or admixture of opposite characters and qualities so as to produce a good citizenry.