Sadly the 1.4x and 2x TC are incompatible with the 18-250 lens from looking at the system. More likely due to the rear element construction on the lens will stick out so cannot be mounted but have not tried it myself so you might want to try it in a camera shop.
With any lens based modifiers, image degredation will take place. The more multipliers that are put in place, the worse the image will degrade by but not enough to ruin a photo. Just do not expect them to be as sharp as a photo taken without a TC.
The other downside is since they also allow less light in, you may lose autofocus depending on the lens being used. I know the only zoom lenses which I can see that will accept the TCs are the SAL70200G and SAL70400G. Wider range lenses do not show on the compatibility info.
Hope that gives you some insight.
