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Most people shy away from undergravel filters.
(just for recap...) They suck waste/gunk/detritus into the gravel bed. As this decomposes, it creates a nitrate factory, so to speak, and saltwater fish are very sensitive to nitrates. In freshwater, where the fish are less sensitive to nitrate, it creates a good zone for bacterial growth. Plus I'm not sure how well it'd work with sand anyways. If you're using powerheads on it, disconnect the undergravel from them (removing it from the tank, of course) and just use the powerheads themselves.
As for the shrimp/clam/mussel thing...most people use the shrimp because it's already dead. No need to kill a lot of life to start a tank, so why not start with something already dead? If you wanted to do it, I don't see why you couldn't. You basically want -something- non-toxic rotting in your tank to kickstart the cycling process.
It's up to you if you want to continue with the fluval. Many canisters also trap debris and detritus in them, creating (yet again) a nitrate factory. In can be useful if you have to run some carbon, or for bacterial growth. I certainly wouldn't recommend buying one, but since you have it, you may as well use it. If you run it empty, just for circulation..it's just an expensive powerhead
If you DO keep sponge/foam materials inside, be sure to rinse them (I'd like to recommend weekly) with _tank_ water (or other dechlorinated saltwater...but why waste new water). This will clean gunk off, so less rots, helps clean the water of large particles, and by rinsing and removing the gunk i talked about, it keeps nitrates from going too high. If you try this, and nitrates are too high (while I'm on that subject, get a good test kit!) just empty it and use it as a circulation device.
About the test kits...
You'll want one to test Ammonia, Nitrate, Nitrite, pH ... Calcium and Alkalinity are an added bonus especially for reef setups. If you add anything else, you'll need a test for those too to ensure you're not overdosing. You should also have a salinity tester. A DeepSix Hydrometer is like $7 and will test for salinity pretty well.
I'd also recommend reading the articles section thoroughly, they're all very good reads, and ask any questions. There are tons of nice knowledgeable people here. I learned everything I know from them.