Is there any negative to putting the filters before the add on lens? Will it still give the same advertised effect that the filters do? Has anyone done this?

If I put a polarizer before a fisheye lens will it help block the lens flares?

You should put the filter in front of the lens or lens adaptor.

Frank is right of course. Sometimes the little thin filters won't support the weight of a heavy adapter with a lot of glass. Could break at precisely the wrong time.