Rih has always praised Gaga, she really likes her and mentions her at interviews and I wouldn't be suprised if that 'one of my favorite artists' was about her.
No, I think it's because they're both the biggest risk takers in today's pop world, and althoug they're both the biggest mainstream artists they can peacefully co-exist without crossing each other.
No even that picture where all the pop girls are looking at Gaga disgusted, Rihanna is smiling and happy. Rihanna and Gaga have exchanged friendly tweets/instagram pictures in the past.