I wear a tie and strongly encourage wearing a tie in the Caribbean, flying frieght. The pollo, shorts, or combination thereof(which is approved) would still result in a strong "flipping out".
Flip-flops/dad sandals, Dad sandals belong in the garbage regardless. Flip flops are for the beach. Both would result in an instant red faced flip out if worn in front of me with the intent of operating any airplane under a company I'm management of. Represent your company like a professional! I don't give a flying hell if it's part 91(like that even matters). If you want to dress like a bum, go work for bum airlines, not for anywhere I'm working. Driving/boating/airplaning, get a damn shoe on! My humble opinion of course.
Based on driving, anything other than a proper shoe is severely inconsistent. Don't even tell me you can drive as well as wearing a proper shoe(an airplane is even worse). I'll, and most, will just assume you're an idiot arguing otherwise. You just are... Some shoes aren't proper for flying either for what it's worth.