This is something I keep coming back around to in this thread.
Why can' the good gods be opposed by Archdevils and Demon Lords? Why must it be evil gods?
That depends on whether your gawds are actual gawds, or merely trumped-up planar rulers. The Archdevils and Demon Lords rule entire layers of the Hells or the Abyss, but those layers are the extent of their rulership (Asmodeus excepted; apparently these days he's effectively a gawd). Actual gawds have influence across multiple planes, and actually have some rulership/authority over things in the Material. Also, gawds answer prayer; demons and devils answer summons. Also, also, gawds derive power (or at least something real) from being worshiped or appeased or propitiated; planar rulers don't--devils, for instance use souls as currency among themselves but they don't necessarily become more powerful directly by collecting more souls.
If the only beings you have that are deriving something real from being worshiped, and/or are answering prayers, and/or are having direct influence on happenings on the Material are good, then the only gawds you have are good. And that seems to me as though something is out of balance.
So, IMO, you need gawds of all alignments, gawds with no alignments, or no gawds at all. I went for the third, because it seemed the simplest; YMMV.