I have sprayed cars with rustoleum with old suction feed guns...never ran any through an hvlp, all mud bog and off-road type stuff. It has been a real long time, but I believe I cut it 75/25 with acetone. I have gotten very good results for the product used, dries fast, and fairly durable for the application. Any paint will get damaged on the trails and spraying cheap rustoleum makes for easy touchups. I shot my bog trucks with real auto paint, but not all make that choice and I can understand the reason.
I am not really sure what your question is other than what viscosity to spray at. Just thin it down until it sprays good is about all I can say; that is what I did. I doubt you will get much help from this forum on this topic. With a bit of creativity you can get the paint to lay real nice. Painting competitors vehicles with rustoleum is what got my interested in paint and body work. I no longer shoot rustoleum or any real "hack" work (I will only use urethane), but by all means go for it. Real body men may laugh, but living your life for others doesn't make it very meaningful.