This is Turtle.
There will be a problem only if the fan blade reaches out under the diffuser to push air up into the diffusers when you have the fan in reverse mode for winter use. if you left the fan in cooling season mode , it would not make any difference.
Now You say you have the diffusers out 3 feet from the center of the room where you would put the cieling fan at. if this is True , they don't make a fan big enough to reach out and cover the diffusers. Now you could be sure by buying the 52" blade ceiling fans.
I will say this it does not make any difference to the diffuser air coming out as long as you don't shoot the air back up to the diffuser by the ceiling fan, but then if the blades do not cover the diffuser then it will make no difference.
The type diffusers you have are called curved air diffusers. they make the air hug the ceiling till it hits the wall and then comes down the wall.
TURTLE