I don't know if I can be much help with Silas....I am also researching the Alexander's in early Warren and Cannon Counties.
In Warren County records, I found a Silas Alexander who was listed in the Militia Rolls in 1810 and 1814. He also bought a land grant in the 10th District which borders on Readyville in what became Cannon County in 1836.
I don't know about him living in Fayette County....but I do think there was more than one Silas Alexander in the area. However, the Silas in Readyville would have been at least around 20 in 1810...so he was born circa 1790 or earlier.
I will let you know if I find anything else on Silas in Readyville.