Very often when one departs from normal word order in English, the effect is to stress something.
Here, the subject complement United is stressed. So United receives stronger attention in the sentence United we are, since this varies from the normal, everyday, mundane, unremarkable We are united.
Down the long, dark, dust-filled road the cat slinked home.
Here the prepositional phrase is moved to the front of the sentence; the effect again is to call attention to it.
Both of the examples above are actually "poetic" in the sense that they depart from normal word order.
In poetry real, (<- see what I did there?), that is, in actual poetry, yes, authors move words around all the time, pulling them out of normal word order. This can be to affect meter (either to stay in tune with it or to depart from it) or it can be, again, to call attention to whatever is not in normal word order.
i thank You God for most this amazing
most is yanked out of its normal place between this and day, and the effect is astounding. Normally it would be this most amazing day.
Hear the poem by E.E. Cummings.
Basically there are no rules in poetry, or other art.