Honestly if you have fishmen, birdmen and centaurs you should be asking how humans havent been wiped out.

All the plains are dominated by highly mobile incredibly strong and fast people that are just as smart as humans but have strong physical advantages and also have opposable thumbs. Humans are better in the woods but thats about it. Goodbye easy farmland in many areas.

The fishmen are dominating the coastlines and can probably prevent easy access to fish and make salt a rarer commodity. They can appear from an environment humans can't really deal with and can attack from it and retreat back in to it where it provides them great cover and unlike dry land, acts as cover from which they can fight. Goodbye many areas of easy coastal fish+salt. Two sources of food down.

And the birdmen are the most deadly of all. They can attacjk you from the sky, manouver anywhere and can shoot you from extreme ranges with telescopic vision and your ranged attacks against them are at a severe disadvantage. And they're faster than you. And they can get at places you can't. Their homes can be up nigh unscalable cliffs and theres no way you can climb to them without getting picked to pieces on the way up.

Maybe I just answered a few of your questions.