The degree of concern depends on how corrosive the streams are to the 316L stainless steel.  As you undoubtedly are aware, 316L stainless steel is less corrosion resistant than the Hastelloy C alloys (assuming you are talking about the Hastelloy C family!). So, if corrosion occurs it will happen with the 316L stainless steel first. In addition, you have the possibility of enhancing the corrosion of the stainless steel through galvanic coupling with the Hastelloy. 316L stainless steel is anodic to the Hastelloy C alloys and small stainless steel parts coupled to large Hastelloy C pieces could cause the stainless steel to corrode very fast. Due to supply issues, we have faced similar situations at my company regarding the mixing of austenitic stainless steel parts with those made of Hastelloy C alloys and I generally do not recommend this. It has been my experience that even if there are no compatibility  issues with the original process, issues often crop up later with new processes as people lose track of what mixture of materials they have installed on a particular set of equipment.