One small point.... Nicaraguans refer to themselves as "nicas" not "nicos" whether they are male or female. As opposed to Costa Rican people who are either "ticas" or "ticos".
As to the use of the name "gringo" in Nicaragua, it can be friendly or offensive depending on the conversation and the intent of the person using it. It
generally refers to people from the USA but many times it includes
Canadians and Europeans especially if the are blonde and blue-eyed.
I, personally have been called gringo by many and various small children who were being playful and affectionate with me at the time. I have also been called gringo by young punks who were looking for trouble or telling me to go home! But my favorite use of the word is when my Nicaraguan
wife makes "gallo pinto" for me. I ask her to add some scrambled eggs, tomato and peppers to it to suit my taste and she calls it "gallo pinto gringo".
And I love it!
Also in Nicaragua and in Costa Rica, it is often a term of endearment to call someone after their appearance. For example, if a person is bit fat (chubby as we say in the USA) they might be called "gordito/a". If they are thin they might be called "flaco/a". Or if they are dark skinned, "negrita or morenita". All with much affection and without offense. Me, I have grey hair and they often call me "chele"! As they do many men with grey hair or are light skinned.