As I recall, rice is not a complete protein source, nor are most dried beans, but the combination of beans and rice is a complete protein source. That may explain why it is a popular food pairing, even before the science behind it was known.
I'm pretty sure that Amaranth is included in one or more of KAF's whole-grain flour mixes, like the Harvest Grain or Special something or other. I love all of their grain blends as part of a bread dough. But I've not tried plain Amaranth flour.
Amaranth isn't a cereal grain and it has no gluten in it, so baking with it would either require including some wheat flour or using gluten-free methods to create structure for leavening to hold.