Another rule stands in the way
It's true that a creature "can only be affected by one polymorph spell at a time." It's also true that the psionic powers metamorphosis et al. are not polymorph subschool spells or even spells.
However, these two facts alone would be insufficient to persuade this GM to allow a creature to employ a polymorph subschool spell and a power in the metamorphosis line so that the a creature benefits simultaneously from both.
The issue is not with the Same Effect with Differing Results rule, though. That rule says
The same power or spell can sometimes produce varying effects if applied to the same recipient more than once. The last effect in a series trumps the others. None of the previous spells or powers are actually removed or dispelled, but their effects become irrelevant while the final spell or power in the series lasts. (Psionics Unleashed 79)
The question is not about using the same power or the same spell on a lone creature multiple times. The question is about changing form then changing form again. Hence it's the next rule that applies, One Effect Makes Another Irrelevant. In Psionics Unleashed, the whole rule is this: "Sometimes, a power can render another power irrelevant" (ibid.). Absent explanation and examples, it's easy to miss.
However, that rule wasn't nearly as easy to miss when it originally appeared in the Player's Handbook for Dungeons & Dragons 3.5. Below is the same rule from that game for commentary and critique:
One Effect Makes Another Irrelevant: Sometimes, one spell can render a later spell irrelevant. For example, if a wizard is using a shapechange spell to take the shape of an eagle, a polymorph spell could change her into a goldfish. The shapechange spell is not negated, however, and since the polymorph spell has no effect on the recipient’s special abilities, the wizard could use the shapechange effect to take any form the spell allows whenever she desires. If a creature using a shapechange effect becomes petrified by a flesh to stone spell, however, it turns into a mindless, inert statue, and the shapechange effect cannot help it escape. (172)
Pathfinder—therefore Dreamscarred Press's material for Pathfinder—is based on the System Reference Document (SRD) for Dungeons & Dragons 3.5, but the SRD omits examples (cf. here and here). And what the question wants to do runs afoul of this rule once the rule's examples are taken into account.
To be clear, in D&D 3.5 when a creature actually changes its form then any ongoing form-changing effect is suppressed. Other form-changing effects are rendered irrelevant precisely because the creature changed forms. The wizard-eagle effectively becomes a wizard-goldfish in the example, not an eagle-goldfish and not a wizard-eagle-goldfish. (Although it may help to keep track of him as a wizard-(eagle)-goldfish, noting the suppressed form-changing effects parenthetically.) This means that a creature can't manifest minor metamorphosis to change form into a hobgoblin then cast form of the dragon I to change form from that hobgoblin into a Medium brass dragon; instead, the creature suppresses the hobgoblin form so that it can change form into a Medium brass dragon.
This is an incredibly important rule in D&D 3.5 where form-changing is a mess. But, because new to Pathfinder is that rule that I quoted in this answer's opening, Pathfinder material usually doesn't have to go on about the One Effect Makes Another Irrelevant rule at any real length (q.v. here). The rule is already a feature of the game's most pervasive subsystem: spells—specifically those of the polymorph subschool.
Anyway, according to the rules, the metamorphosis powers' effects suppress or are suppressed by any other form-changing effect as described in the Player's Handbook, above. So you can totally play it that way. But, still, I can easily imagine a GM writing a polymorph descriptor mirroring aspects of the polymorph subschool and sticking that descriptor on the metamorphosis powers and similar powers just to fully excise from their Pathfinder campaign any vestiges of painful 3.5-style form-changing.
"But it's just a cosmetic change!"
The minor metamorphosis power says
You channel your psionic power, snapping bones and twisting flesh, transforming yourself into a new shape or form. You can take the form of any creature of the same size as yourself, but this is a purely cosmetic change, your abilities, powers or stats do not change beyond the options selected below. (Psionics Unleashed 159–60)
A transformation that snaps bone and twists flesh is in no way purely cosmetic as a traditional reading of the word would suggest, which is why the author followed up that statement with items that are not cosmetic: game statistics. A creature that has employed a metamorphosis power has, in fact, changed form even though for the most part the creature remains mechanically unchanged.
