I happened to see the exchange you had with Judie Flint regarding your Alexander Monteith.Judy and I are 3rd cousins.While it does seem quite possible that there is a connection between our two families, I think it's unlikely that William and Alexander are brothers.
William Ruthven Monteith was born in Kilsyth, Stirlingshire, Scotland on 5/29/1814 to William and Margaret (Ruthven) Monteith.His father and mother were married there on 11/28/1813 (must have gotten an early start on the baby!).
It is possible that William was married before - he was borm in 1785 - but, if so, there hasn't been any report of it in our family.It is also possible that the link is in the previous generation - William was the illegitimate son of William Monteith and Mary Finlay so perhaps his father had a legitimate family as well.
Is there any mention in your family of Kilsyth?Or of family members emigrating to the US?Please feel free to get in touch with me directly if I can be of any help - firstname.lastname@example.org.Good luck with your search.