crystal swapping questions

Hello all,
I finally saw my first Omega on someone's wrist, it was very nice looking. But the thing that really grabbed me was how amazingly clear the crystal was. I'm assuming its because of fancy (and expensive) AR coatings, but it just made the dial look so cool compared to the crystals im getting on Aliex cases. I have bought a crystal press, which I've never used. It feels like swapping crystals is definitely next level compared to the watch assembly i've done so far, so I was hoping for as much info as anyone is ready to share. Specifically:

1. Any advice on crystal sizing?
2. What's the deal with gaskets?
3. What product descriptions of crystals in Aliex actually equate to a super clear crystal like I saw on that omega (or is that even possible)?
4. Anyone ever bought from Perrin in Toronto? Their product descriptions are almost non-existant and the prices swing wildly from one crystal to the next, but they have a lot of selection. . .
5. And anything at all you've learned in the crystal swap game. Thanks so much!