That sounds fine to begin with for water changes. Remember that cephs can grow fast, so just keep an eye on the nitrates and adjust accordingly.
RO water is a necessity. There's enough copper in our salt mixes, we shouldn't be introducing it, or anything else, from our water! Yeah, some parts of the country have really good water, but I don't think its worth it. I spend between $5-10/week buying RO water from my fish shop, which isn't much compared to the cost of feeding my cuttlefish
That damsels can hurt octos is a big surprise. Reefers have a stereotype about octopus because they can come into their tanks as hitchhikers and eat a lot of things. They do that, of course, but the reality is more complicated. Since they don't have a protective shell they really use their behavior as a first line of defense. If there's something in the tank they're not sure about, they're happy to stay in their den all the time because that's what they're used to in nature. It all comes down to personality.
I agree with the earlier comments about keeping mimcs. Obviously because the animals aren't well known to science and their numbers appear to be diminishing. They don't seem to make great pets, few people have had luck with them. Wild-caught cephs from the other side of the world go through a lot of stress and most end up dying. Wild-caught cephs from around the US seem to make better pets.
There are people around; mainly on RC where the signal-to-noise ratio is so low; who talk about keeping mimics. IMO its the mentality of getting something in a 10 gallon tank for a few weeks to make the dorm room look really cool. Shouldn't keep these things if you're looking for a decoration. That's what freshwater is for
Dan