Now that you mention it, you're right about all this. I have noticed many people who don't want to include someone into their roleplay all because they weren't playing the same species as they were. With me and many of my friends, it's never the case. I could be playing as a fox, and they would be playing as cats. Or they would be playing as a hyena, while some of us are playing as foxes and cats, etc. There have been times when my friends would be roleplaying as warrior cat characters and I would be roleplaying as a fox.... and some random people would come up to us and tell us that cats shouldn't be friends with foxes, vise-versa. It's crazy I tell you.
I see nothing wrong with people roleplaying with someone who is playing as a different species. Animals of different species interact with each other in the real world, but it's not always an enemy issue. I guess most people on this game want to play "realistically" and only participate in a roleplay where everyone is of the same species; and all other animal characters are considered enemies or prey.
It's human nature to be part of a group of individuals who share similarities. It's like the stereotypical cliques in middle school and high school, for example. Jocks like hanging around with other jocks, nerds hang with other nerds, cheerleaders with cheerleaders, the fashionistas hang with other girls who are fashionistas too, emos cling to other emos, so on and so forth. And in most cases of middle school/highschool society, most other cliques aren't supposed to mingle or talk to each other. And if someone were to talk to someone of another clique, someone would say, "Why are you talking to him/her? He's/She's not one of us." I'm not saying it's not the way things should be. In fact, I was one of those kids in middle school and high school who didn't mind talking to anyone of any little "clique". There are people in this world who don't mind socializing with people of other "cliques", backgrounds, beliefs, cultures, ect. I applaud them for not being discrimative.
When it comes to roleplays in FH, I often times wish people would be more open to new ideas such as roleplaying with characters that are of another species. I have RPed in some interesting roleplays that involved speices mixing together to create a good story.