I lived in Edinburgh as a student during the early 80s, & being a footie fan, I obviously "adopted" a Scottish club for the duration, happy that there was no conflict of interest or loyaties with my own team (Sunderland).
I knew that I couldn't pick either Celtic or Rangers, on account of being (a) English and (b) resident in Edinburgh, so I actually picked Dundee United. This proved a good choice, as they, along with Aberdeen, won loads of trophies over the next few years.
However, I used to visit Glasgow from time to time, and did also take an interest in Old Firm games. On one occasion, I accepted an invitation to go to one. It was the most terrifying experience of my life, and still remains so more than twenty years later. I have often wondered what would have happened if I had opened my mouth & revealed myself as an Englishman in that crowd.
I've often heard it said that Scotland has no need for racism, as it has bigotry instead. I used to think that was a joke, but I learned that day that sectarianism is alive and well in Glasgow and on the West Coast.
Despite all that, I would also say that the years I spent living in Scotland were wonderful, and with only one or two exceptions, I was made to feel very welcome. You can find anti-English sentiment if you want to go looking for it, (or go provoking it) but the vast majority of Scots are great people IMHO.