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Personally, I recommend a 5g for a dp. A smaller tank could probably work, but they are usually about the same price as a 5g anyway. Dp do not need brackish water. You can get them from a fish store or online. It depends on the temperment of the fish if it will be ok with shrimp or not. Some are, many are not. You can always test it with a few ghost shrimp.
5G plus, preferably heavily planted for 1-2. They are FW fish. They are usually available online and often in individual LFSs. Be sure to ask if they've been dewormed & QT'd. Any invertebrate smaller than or equal in size to them will be a goner. Larger shrimp may lose legs.