Can a convicted felon be a reliable witness

There is no classification of a reliable witness. The prosecution can call ANYONE to testify, and often uses convicted felons in their case. It is up to the defense counsel to impeach the convicted felon's testimony on cross examination. This is normal in trial, where there are co-defendants ratting on each other. You need an experienced trial lawyer with a history of actually "going to trial" and not just taking your money and pleading out.