@raddcuban
I've never heard of Roman cav being made up of slaves. Of course they were nearly always from conquered nations, but they were still free men.
It's a moot point though since slaves have often been given very important roles both in armies and in government like the Janissaries.
And I think that, while the are named after the Germannii, I think that was since Germans are well known and he could get the blonde hair color that way. I think the actual horseriding skill is based on the steppe nations instead, who were much better at horse riding than the Northern Barbarians. A place like Sarmatia or similar. Especially given that all the nations have exhibited a Greek-style culture. That existed in Italy, but it was spread all the way to India as well and, given other cultural signs, it's likely this story is based on the eastern part of the mediterranean rather than the western. And those steppe nations were right above the eastern Greek empires unlike the Germannii which were on the other side of the known world.