For example (parting ways now) I consider the ease with which
breeders have grown feathers sticking out from scales in a few short
decades (centuries?) to show it's really not challenging to produce
an intermixed dermal type with feathers (or fuzz) and scales. Yes,
it's very uncommon in extant birds, but I consider that a derived
condition that saves on metabolic cost (birds have long since lacked
a need for scales on most of the body, so why not make sure there's a
gene to prevent them from being grown where they aren't needed).
As an analogy consider avian beaks and teeth: Tooth placodes have
been famously shown to still be viable in extant chickens, just not
in their beaked mouths, where they are actively suppressed. Given
the developmental genetics of extant birds one could conclude that
it's therefore impossible for beaks and teeth to co-exist in the
same animals mouth...after all, the presence of beaks turns off
tooth development! But of course in this case we have an extensive
fossil record that shows that teeth and beaks co-existed in many
dinosaurs (including early "birds"). We therefore accept that there
were several evolutionary "experiments" with various configurations
of tooth placement and morphology that coincided with various
configurations (and morphologies) of beaks.