Instead of buying sponges from a non-pet store, I'd recommend buying the filter floss, or something of that nature, from the pet store. I've been using $5 worth for over a year running a Penguin 125. If you replace the retail Penguin filter media once a month, as they suggest, its going to break your bank. (I've also been removing carbon from my Fluval 404, in favor of the heavy floss. I know carbon eventually loses its main properties, but it still acts as a refuge for beneficial bacteria. I figure the thick floss will do the same job at a fraction of the cost.)
All my tanks have redundant filters. One cannister + 1 hob for the larger tanks, two mini-hobs for the 10g. I was more worried about dead zones in water movement than anything else.
Both my 20g and 10g have more fish than most people would suggest, but, I have both tanks stuffed moderately heavily with plants. Mostly java fern .....gotta love them, just put a small lead weight on their root and they'll grow anywhere.
Regarding the bio-wheel affecting plant growth; the bio-wheel creates a lot of aeration and disturbs the surface of the water, so carbon dioxide is released into the air instead of staying in the water for the plants to feed on. Otherwise, the bio-wheel is great for the fish. Aeration + bio-filtering. I recently removed both bio-wheels (since I have plants).....been watching the fish carefully to make sure they are getting enough oxygen. Its only been 24 hours though.