Is it okay for guys to wear these? I want to order them in size XL

It is completely fine for guys to wear thongs. These are high quality and they look great. Womens thongs are much better because they don't have a stupid pouch provides no support at all. I'm pretty small and I have tons of room in these. They are perfect for any man who is average size or below.

Im a mtf preop transgender individual will these fit

I'd assume so, I've had two kids so my hips are a bit wider and these sit pretty high on the waist so if worn lower accommodate the parts you have I'd think it'd fit ok. It's a very comfortable thing, elastic is wide and holds shape well. I can't be 100% obviously but I think it'd fit you ok

I am 5'4 and I have a curvy build. I usually wear a 9-10 in pants, in Victoria Secret underwear I often buy smalls. Should I buy a medium or small?

I am 5'11 and wear a pant size of 2 or 4, I also always bought a small at Victorias Secret {they don't carry my favorites anymore}. I went to Dillards tried on a pair of small - fit absolutely perfect, then I went on-line and bought a ton cheaper than Dillards. Hope that helps :)