Definitely some rivalry between the two ports but, as you mention, in my time don't recall it spilling over in to the football; of course there may have been banter between those who interchanged between the two ports at different points similar to any workplace now.Shame you couldn't hold on SOTC.
I thought there was some historic animosity between the Cities due to Naval rivalries, if not between the actual football clubs.
As you well know, "we are the Argyle haters!!
@Rosencrantz is absolutely right that the gloves may well come off when it comes to the field gun (that picture has made me smile more than you will know!)
On a league one tangent I reckon one of Tisdale or Deadly Derek could well end up in charge at Bristol Rovers. Nearly had a punt when Tisdale was about 20/1 earlier in the day and wish I had now - though wouldn't be surprised to see them appoint another coach in the Garner mould.