Ferguson/Craig Connections

[janmacD]

Trying to find the birth of my Grandfather in Rothesay. He was later known as David Ferguson.
All David Fergusons born around the time in Bute have drawn blanks.
I'm currently looking at twins David and James Craig born 1881, Rothesay. I suspect David may be my Grandfather (Later, David Ferguson) My theory is that he was 'adopted' but there appears to be no record of adoptions.  I can't find the twins together after their birth.  I see James possibly adopted by Mary and Colin McPhail in the 1891 Census.  (Mary's maiden name was Craig so an aunt, maybe?)
The name 'Craig' crops up later in my Grandfather's first daughter's name in 1920 (Isobel Craig McInnes) which suggests a connection and my Grandfather always claimed to be born in Rothesay, Buteshire and would often visit 'family' and friends there in later life.
I'm looking for a CRAIG family connection to Duncan and Catherine Ferguson in the Lochgiphead/Kilmartin area at that time which might convince me that this is my Grandfather. 
The earliest link to him as David Ferguson is 1891 census aged 9 at Ford, Kilmartin.
Any suggestions or search ideas will be very welcome.  Thanks!