Will,
You can never have too much filtration. You CAN have too much current in the tank but that isn't possible using sponge filters. I use two Hydro IV sponges in my 60 gallon grow out tanks. They are each rated for 80 gallon tanks, so I have filtration for filtration for a 160 gallon tank in a 60 gallon.
Your bacteria colony will only get as large as the bio load placed on the tank.
It's really a personal choice I guess.
P.S. Take a look at Jehmco for sponge filters or Kens Fish. They both have great prices on sponges!