The electrical engineer in me would look at FeCl3 as source of FeIII, but chemists might know better. The trace quantities of FeIII you are going to add make it unlikely that the Cl- has any effect on your developer's properties.
Ferric chloride is a poor choice for making a solution of known ferric ion concentration. It is one of those compounds of indefinite composition. It hydrolyses in moist air and loses chlorine. It is also deliquescent. So there is no exact composition. The three compounds I suggested in a previous post are all stable and have definite compositions.
A rock pile ceases to be a rock pile the moment a single man contemplates it, bearing within him the image of a cathedral.