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