Conductivity ~~ Hardness, buy a good meter it will make your days a lot easier. I use the Hanna Combo meter HI 98129, pH & conductivity, and temp too.
Estes marine sand comes in white and a beige, but it will buffer your water. The colored ones do not, but the white and beige do! Different lighting may give you the desired effect with sand color. Get bulbs with Kelvin ratings in the 9,500 to 11,000 range.
Yeah, 2217 is a great idea.
Oscar vs. Goldfish, goldfish are coldwater, they are not going to thrive in a pond that is cycling for a ray that is maintained at 78-82F.
The other option when I do this is just to cycle a filter and then do small water changes daily once the ray is added, just to curb spikes in ammonia. Also I would get a good aerator and just run a couple of hydro sponges in the 90 now for a month and move them over with the ray, great for bio filters. They can the be removed from the pond in a month or so once the main filter is up to speed. Remember that the main pond filter needs to seed and in that process you will have ammonia spikes as the bacteria colonize the filter bed. In the case of rays I personally feel more comfortable cycling a new system with fish, big fish that like to eat a lot and are hardy enough to take some ammonia. Catfish are good too long as you have a place to unload them after you are done using them! Oscars I find are just easy to unload on stores, give away for free and it is a win/win for you and the store.