Actually, i have read richard grey's diary, and completed fallout 1 and 2.
The deathclaws haven't been exposed to 'pure' FEV. They have been exposed to the mutagen after it has been affected by the radiation in the wastes, or possibly from drinking sources of the FEV. According to you, only humans have had direct contact with the FEV: if this is so, how the hell were deathclaws created in the first place? It would require a very large prolonged dose of the FEV to cause such a mutation, and no one 'created' deathclaws deliberately (or, not at least to current FO knowledge). By comparison, the increased intelligence was a 'small' step. Okay, i was wrong about the random effects: Although by reading the description of the FEV (can't remember where) it did make it seem like intelligence should be increased across the board, regardless of species. This is due to the consistent doubling of genetic material by adding strands of RNA.