I don't know if using too many polyfilters would hurt... but I wouldn't think so. Maybe someone else can offer some input on that one!

I should have asked before - what size is the tank and are you running a skimmer on it? If you have a skimmer, I wouldn't think you need to use a polyfilter at all - you could just use the filter to move water for you. (I don't have a skimmer, so the polyfilter keeps my nitrates under control.) If you don't have a skimmer, one polyfilter would likely be sufficient for a small tank, just check it often to get a feel for how long it will last in your tank. I change mine about every two weeks.

Hope this helps!