If you wanted to you can use both. Since the plastic you're painting isn't flexible, an adhesion promoter then a regular primer would be fine.
+1 on the flexible additive, mainly used for flexible parts such as bumpers, etc. As for adhesion promoter/primer that depends on the reason for the primer. Do you need to fill scratches from sanding? Fusion is designed to adhere to most plastics without an adhesion promoter or primer. If you do need a sand-able primer/surfacer than yes an adhesion promoter will be required. I have used Fuson on a number of motorcycle plastics (mostly fairing pieces) with good results but have seen others have problems on things like ATV plastics. Best adhesion is generally straight to the plastic (no primer) if proper prep is done. The piece needs to be spotless and any sign of release agents, out-gassing etc removed. Cleaning with soap and water then a mild solvent such as mineral spirits or distilled alcohol (test a small spot first for softening) should do it. Unfortunately there are so many types of plastic in use today it is hard to make generic recommendations. Check Krylon's website for specific instructions. Hope this helps.