It's doable depending which 'dwarf' you get. Cr. compressiceps would be your best bet as they max out a 3.5" You might even be able to keep some larger community fish with him such as Tiger Barbs or Von Rio Tetras. Cr. compressiceps is an insectivore with a relatively small mouth (for a pike) so is less likely to eat other tankmates.
Many of the other 'dwarf' pikes are not so dwarf when it comes down to it. Dwarf is a relative term. For example, Cr. regani is considered a dwarf yet can grow to 6". 6" is only dwarf when compared to a 24" Cr. sp. 'Venezuela', but 6" is still a good sized fish. These would need to be kept by themselves in a 20 with the exception of maybe a Bristle Nose Pl*co. Still a very neat fish, just larger than perhaps expected.
Most importantly, in a 20 gallon tank you will only be able to keep one pike regardless of which species you pick. Even the small Cr. compressiceps will not tolerate another pike in such confined quarters.