Foederati and the Roman army of the C5th are a very tricky subject IMO as we just have so little information to go on. Even something like the oft mentioned (in modern works) Gothic settlement of 382AD is something we know virtually nothing about other than Goths settled within the empire - there are no real records of what the settlement actually was and what it entailed despite what is written in a lot of modern books.
Richard has given what you might call the top down view of how the list was done, and we decided to stick to a fairly traditional approach.
If I were to write the list again I'd probably do it differently (Richard might not though

) and also probably change the cut off date between the Dominate and Foederate - I'd also be tempted to include Alaric's "Gothic" army and its successors to 419AD in the Foederate list. But that is rather hypothetical.
I'd make one comment on the original post on this topic in relation to:
Barbarian Armies in this period (AD 451) like Early Ostrogothic, Later Visigothic and Gepides can already field 'armored' Cavalry!
In that those Germanic armies mentioned have a very limited number of Armoured cavalry which, if we are being honest, are pretty speculative for a mass battle situation.