Single stage for me. I used dupont rival over the winter with good results. It laid on smooth with my devilbiss finishline and if it werent for all the dirt in my garage, I wouldnt have needed to wet sand or polish it. Pretty much i'd recommend to stick with industrial finishes;
Dupont Imron, Dupont Rival, PPG Fleetline
Everyone I know has their own preference. I tried the Rival which is supposed to be a low VOC replacement for Imron. It seemed to work pretty good. I was told by a local supplier that Imron didn't have metallic single stage, but Rival did. Someone told me later on that was false. I had some issues on the bottom deck where it was peeling up from the primer. The paint seemed to bond well to the primer, but the primer didn't bond well to gelcoat. Maybe I should've scuffed it heavier, or just used epoxy primer. The top deck I painted right over the gelcoat and have no adhesion issues.
As for reducer, I would recommend it. Dupont spends tons of money to formulate a perfect product for fleets and OEM refinishing. Some of their line ups have been around for over a decade, I think by now it should be perfected. If the product information sheet says to use reducer; use it. Often times they give options of reducer ratios for different uses. Like if you were going to spray out of a HVLP, pressure pot or brush.
Whatever you spray, take extra precautions with ventilation, cover skin and use a *new* respirator meant for it. All these paints have cyanide in it, some people can handle it and wont notice the effects immediately. Others can have an allergic reaction instantly.