Rickless' implicit argument depends crucially on the claim that something of which many things can be predicated is itself multiple. Where does this claim come from? Rickless finds it in the Philebus, as he indicates in his 1998 Phil Review article:
Socrates has already acknowledged that something is many if it has multiple parts, and there is evidence in the Philebus that he accepts that something is many if many predicates are true of it. (522)
It should be noted that, in the Philebus, Socrates accepts not only that a man’s being many follows from his having many parts (14d-e), but also that a man’s being many follows from the fact that many predicates are true of him (14c-d). (513)
But there are at least three difficulties with this.
1. It's not clear that Plato even endorses the relevant principle in the Philebus. In 14c-15c, Socrates distinguishes two sorts of problems involving one and many. He dismisses the first sort as merely eristic and "no longer even worth touching; they are considered childish and trivial but a serious impediment to argument if one takes them on". The second sort he takes seriously: "But when someone tries to posit man as one, or ox as one, or the beautiful as one, and the good as one, zealous concern with divisions of these unities and the like gives rise to controversy". The latter sort seems to correspond to the kinds of difficulties that are raised in the first half of the Parmenides. (Look here to follow out the text, if you wish.) But it's within the first group, the sort that 'is not worthy of scrutiny', that one finds the eristic argument that Rickless is appealing to, that "there are many 'me's' , and even contrary ones, when [someone] treats me, who am one and the same, as tall and short, heavy and light, and endless other such things". The passage hardly gives us grounds for holding that Plato was sympathetic to such arguments or accepted Rickless' key premise.
2. But even if Plato was endorsing the principle in the Philebus (which I don't grant), why would we be justified in presuming that he endorsed it when he was putting forward, as Rickless calls it, the "Middle Period Theory of Forms"? That seems an unjustified leap.
3. But even if one could infer that the principle was at work in Plato's "Middle Period Theory of Forms", because of something said in the Philebus (which I don't grant), it's not clear that the principle would be relevant, when the many predicates involved are all the same! On Rickless' understanding of the TMA, each Form of Large participates in an infinite number of Forms of Large, and, because of this, multiple predicates become true of each Form. However, each of these multiple predicates is, simply, 'large'. Thus, on Rickless' interpretation, the important point is that we can say of the first Form, as well as each of the others: "This is large, and large, and large, etc." ad infinitum. But it's at least not clear that our being able to assert multiple predicates in this way would imply multiplicity, on the same grounds that saying 'tall' and 'short', or 'heavy' and 'light', might be taken to imply multiplicity. (Oh, sure, one might want to maintain that each predication of 'large' is distinct: ‘This is Large1, and Large2, and Large3, etc.’ But then that argument has to be implicit in the passage as well!)