I have read and heard over and over again. You can never overfilter a tank. Of course, like mentioned already, you dont want the current to be too strong but other than that, more the better.
Of course, in regards to the biological filteration, the bacteria culture will only grow as big as there is a demand for. Meaning, lots of fish, bigger bio, less fish, smaller bio. Where the over filtering comes in is the mechanical (and chemical if you so prefer). And of course it seems that you can never have enough mechanical filters. You will notice a difference it water clarity if you can 'polish' the water with floss filter media (or other fine media).
As for cleaning a filter, my lfs (He's the best btw) says it depends on your nitrates and filter flow. The nitrates part I believe he's referring to the big up of junk in the filter and therefore causing ammonia, nitrites and nitrates. In my 90G tank, I have no nitrates (I have real plants) therefore he recommends not cleaning the filter for a couple of months. Unless of course, the filter starting plugging up and affecting the output.