Yep, Animals are definitely going to have certain things in common due to migration, especially water ones, since they can move everywhere.
As for the Gravity, the Inner Core can spin fast while the surface of the Torus has a more normal Speed, so you dont notice much of a gravity difference on the inside compared to the outside, except maybe being slightly heavier. And the Inner Core itself can be made of a very dense Material, so that the Ring itself has Mass based Gravity.
The Ring itself could also be closer to the outside of the Torus when it comes to its center of Mass, increasing the Gravity on the outside by the amount of centrifugal Force that is exerted on it, equalizing the Gravity in return no matter where you are.
Also Centrifugal Force is a proper term, nit-picky people are just too lazy to transform the Vector into the direction they want it to be. Every Force has a counter force if nothing ends up moving away. 