Given that plenty of Greeks are, to put it mildly, less than fond of Albanians, is there a lot of tension in the Greek Diner/Pizzaria run by Greeks, and those run by Albanians? I hope not, but I’d guess that there is.

Sure, there will be some greeks that don't like Albanians, for x or y reason, but it is not universal.

Also, there substantial number of greeks with Albanian origins: (https://en.wikipedia.org/wiki/Arvanites ) and Albanian and greek food is very similar as there has been a lot of cross cultural pollination.

Both Albania and Greece were part of the "same empires" for millennia. First the Roman Empire, then Byzantine Empire, then eventually the Ottoman Empire.

Even before that they were either in alliances, or fought each other. Alexander's the great mother was of Illyrian origins.