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Depending on what corals your keeping you very well could over skim the water. Sps isn't an issue but softies actually do need trace levels of nitrate to build their tissue. A good rule of thumb is get a skimmer that is rated for double what your total water volume is (for example 40 breeder with 20 sump would do well with a skimmer rated for 120gallons)
Ya you should be fine then with a skimmer rated for 400l... Even if you would buy a monster skimmer that could potentially over skim the tank all you would need to do is tune it down a bit. This gives you the option of not having to buy a new skimmer for each upgrade in size.