Terraria has Corruption/Crimson/Hallowed Mimic Chests that are like mini bosses, 8000HP on Expert mode. The wiki claims they only spawn in Crimson/Corruption, or Hallow. Obviously it's around cavern level, but does this apply to a corrupt/crimson/hallow Ice Cavern? The wiki does not make this fact apparent.
Yes, an area need not only be a single biome. Take purple ice for example, it's part of both the tundra and the corruption. Although you don't need these 'combination' blocks to have multiple biomes. Having enough of each in the same area will do the trick.
If you're in an area that is in multiple biomes any monster that can spawn in any of them can spawn there.
Many grinders use this to have a single grinder that is in up to 5 biomes at once. (Usually Jungle, Ocean, Corruption, Crimson and Tundra)
Note that a biome cannot be corruption/crimson and hallowed.
Also note that if you're looking for mimics, it's far easier to spawn them then to hope for them to spawn. You can spawn them by placing a key of light or a key of night into an empty chest. They can be made at a work bench with 15 souls of light or night respectively.