If Monkey was not very concerned about the visitor cat then that is a pretty good sign to me. Some cats love company and others would be just as happy being the only cat. I have one of each
. I think Desi would prefer to be the only cat in our house but for the most part her and Fred have achieved a harmonious relationship. They will never be cuddle buddies and have the occassional minor scrap but they do actually race around the house and play. My point is that even a not super social cat can benefit from another cat as long as the other cat doesn't super stress out the other.
I also feel much better when we occasionally leave them for the weekend (neighbours feed and check on them) knowing they have each other for a bit of company.