In my humble opinion, it is far better to over filter your tank than to filter "just enough".

Even if your filter is "rated" for a 55 gallon tank, chances are, once it's clogged with lots of good bacteria, the water flow isn't going to be optimal. Remember, the specs quoted are under absolutely perfect conditions.
I have a 75-Gallon tank, currently housing about ~30 African cichlids (some fully grown), 3 Plecos, 2 Whale Catfish and 3 Synodontis Batensodas. And, if I go according ot the specs of the filters, I could get by with just a quarter of one of the two filters I have. Instead, I'm currently running these:
--> Rainbow Lifegard 300, Fluidized Bed Filter - Rated for tanks up to 300 Gallons.
--> Rena Filstar xP3, Canister filter - Rated for tanks up to 175 Gallons.
In my experience, there is absolutely no bad side to over filtering your tank (except for perhaps the extra maint time and initial cost up front). If nothing else, you'll have a backup filter if one craps out.
