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As long as they are not listed in a tier higher than NU. For example, Scyther must be used within the UU level restriction, but Haunter can be used at level 100.
At first, I didn't want in, because I know I will lose (I suck at NU and Ubers). But then I thought "Why not? What's the harm? Does the world end if I enter a tournament that I can't win?"