Factual error: The woman walking past waiting fares, saying "excuse me, this is important." She gets into the right rear door of a 1963 Chevrolet, with flat door sill. In the interior of the car there is "Kick Up" in the doorsill, and a Ford door release handle, flush with the side trim. The 63 Chevy had different handles.
Factual error: When the pimp murders the hooker in the taxi, he is said to have forced drain cleaner down her throat. However, he uses an aerosol can. You can even hear the metal ball shaking inside of it several times. In the 1970s, drain cleaner did come in metal cans, but what he is using is a spray can of some kind, which drain cleaner did not come in. It is always in liquid form to pour down a drain, spray would not release enough.
Answer: Because Davis and Sweet demonstrated keen interest, skill and knowledge of firearms (particularly of Harry's .44 Magnum) when they first met. They were flattering Harry, probably in anticipation of inviting him to join their death squad. Harry was impressed and asked for them specifically, even though they were rookies, but he didn't suspect them of being the vigilante killers at that point.
Charles Austin Miller
Yes, he did suspect them. In fact, he suspected Davis from the moment he found out that Davis was the one who reported Charlie McCoy's death. He confirmed his suspicion about Davis after he compared similarities between bullet that killed one of the victims and the one he purposely misfired from Davis' revolver during shooting competition and retrieved it afterwards. He explained it all to Early Smith right after raid on Palancio (and thus unwittingly sealed his partner's fate).