I am not sure of the weight conversion, but I use 11/16's for canoes with "standard" rib and planking thickness, and say 1/2 inch for lightweight canoe with thinner planking and/or ribs.
The 12 oz tacks are the 11/16" tacks which are by far the most common canoe tack for most canoes. 6oz canoe tacks would be 1/2" in length and used on the lightweight canoes.
the 7/8" long tacks are 18oz. The 14oz must be 3/4" long which I would consider to be a tad too long for most canoe work. If the tack is too long, it has trouble forming a proper clinch, the metal will bend and split the rib or planking. You might be able to use the 14 oz in place of the 12 oz but they might cause some problems in some areas.
I picked these up from a known builder so I assume the 14’s should be fine.
Rib thickness up this way is generally 3/8th I think as opposed to 5/16th. That should help a bit.