Yeah, the pure RO is definitely <7 pH. I didn't realize it was hard to modify!
re: crushed coral, actively considering it, yes. I'm thinking I'll put a bag of crushed coral in the filter, and keep pre-mixing the RO and buffers for water changes. I assume this is necessary so that I'm not dropping pure RO water in at water change.
I'm planning to go with guppies, no specific type (NOT starting with a $60-per-trio type). I was hoping to get pH to 7-7.5, but if ~6.6 is ok then I'll consider moving on from concern to happy dance.