The 1871 Selkirk marriage certificate says Phelin was a labourer. In general, the folk listed in the tithes were farmers. The tithes were in effect a tax based on the amount of land you held, excluding anything less than about an acre. So labourers generally weren’t listed in the tithes. I wouldn’t rush to assume the Phelin in those records must be your family. It probably isn’t.
Griffiths did list many labourers, as well as farmers and so can be more helpful. Farmers will have a bit of land (measured in acres, roods & perches), labourers generally just a house and a few perches, if that.