The Defendant in the Case of People V

The defendant in the case of People v. Thomas was convicted of the murder of his wife and her boyfriend. On the witness stand, he claimed that a witness with whom the defendant had served time was incompetent to testify against him. Defendant Thomas alleged that the testimony of the former inmate was simply too incredible to be believed, partly because the former inmate referred to the defendant's deceased wife by the wrong name and also indicated that the murder weapon was a statue when it had been a piece of wood with a nail in it. Is a proposed witness generally considered competent until other factors indicate otherwise? What is the usual test for determining a witness' qualification to testify? If there are some problems with the testimony of a witness who is deemed competent by the trial judge, do those problems or inconsistencies affect the weight of the testimony?