400w is definitly overkill, you would just be able to keep the most light demanding corals in the upper portion of the tank and try to shade others down lower. 400w halide will definitly throw off alot more heat than a 250w, but your definitly going to need a heater either way, to help keep your temp consistant.
a 150w or 250w halide will be more than enough light for a 20g.
As for your livestock and equipment list, i don't see any problems (just might be a little heavily stocked, so get a good skimmer) And make sure you pickup some good test kits (calcium, alk, nitrate, pH, etc.)
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