When I spoke with the RO company they did not request any preparation. Their biggest concern is getting the CYA down, as their machine uses TDS to determine when the water is "clean". They mainly deal with high CH issues in our area so are not completely sure how CYA acts on TDS. I was not planning on adding any more chemicals to the water until they come in a weeks time. My question was more about what other tasks would be useful to do. For example cleaning the calcium deposits my thoughts being that any spill over into the pool will be removed by the RO.
As for draining the pool, I estimate that would be much less expensive than the RO, but my area is low on water, so I don't it is reasonable to dump 20,000 gal of water.