Everyone knows that if you trying to represent the Earth at the size of the tennis ball for modeling purposes, any claimed rotational speed for the Earth would be represented as FASTER as the size of the model DECREASES.
That depends entirely upon what you are trying to model.
But thanks for admitting all those spinning the ball at 1000 mph are full of shit.
If you just want to focus on the centrifugal forces then you note that the apparent acceleration is given by a=v*omega=v^2/r=omega^2*r
So if you go from a massive Earth to a tiny tennis ball, you need to reduce the linear velocity and increase the angular velocity.
Scaling the radius by a factor of 1/1000 000 will require scaling by either 1000 or 1/1000 (depending upon which you scale).
That means the rotational speed would go from 0.25 degrees a minute up to 250 degrees per minute, still quite slow, not even a revolution per second, while the velocity drops from 1000 miles per hour to 1 mile per hour, quite slow, slowing than walking pace. But that is for a 6.4 m radius object, so still larger than a tennis ball.
Dropping it to a 6.4 cm object, (i.e. radius scaled by a factor of 1/100), you need to scale the angular velocity by 10, increasing it to 2500 degrees per minute or roughly 7 revolutions per minute or 42 degrees per second, and the linear velocity by a factor of 1/10, or dropping it to 0.1 mile per hour, 0.16 km/hr or roughly 4.4 cm/s (if my math is right).
It is nothing like the tennis ball spinning tens or hundreds of times per second.
However, if you wanted to compare that to gravity, you also need to note that the force of gravity would be reduced.
If you keep the density constant, then M=p*4/3*pi*r^3, so ag=G*p*4/3*pi*r^3/r^2, which can simplify to ag=k*r.
Notice how this is proportional to r, just like the centrifugal acceleration in the form of omega*r.
So if you wish to compare centrifugal force to gravity, you keep the density constant and simply scale R. Omega remains the same.
So if you want to compare gravity holding water to a spinning Earth overcoming the centrifugal forces, to doing the same on a tennis ball (a metal ball would be better for the density), you spin it at the same angular velocity.