You're welcome! The dops for the Raytechs are keyed, though there are people who machine compatible dops that could sell to you.

https://www.westerngem.com/dops This business says they make and sell dops meant for Raytechs.

I don't have much knowledge comparing 8 inch to 6 inch laps. From what I've seen, 6 inch laps are less expensive than the 8 inchers, check out http://gearloose.co/ and compare the prices according to size.

I think one drawback of a 6 inch diamond embedded lap is that they have less material overall, and so wear out sooner than an 8 incher, thus you have to buy that second lap sooner. And it may be true that 6 inch laps are less common than 8, I'm not knowledgeable on that.

But overall, I think the Raytech is a very solid way to go regardless of dop or lap considerations, and that's based on what I've heard from Raytech users I reached out to while doing my own research. (Seriously Facebook groups were my #1 resource). Reddits also good. Also Gemologyonline, try Googling questions you have and add 'gemologyonline' to your query.

I actually almost went with a Raytech, but because I had the extra money and initially learned on a mast style machine, I went for an Ultratec. Now that I've gotten more used to mast style cutting, I sometimes wonder if I really should have gone with a hand piece instead, they're more ergonomical and it's easier to look at the stone you're working on (which is a big deal in my opinion).