Peat doesn't really soften water, it acidifies it.
Generally it's placed in a filter bag and inserted into the filter. It doesn't last forever, how long depends a lot on the filter and the tank size. Generally, you don't clean it.
Some SA fish can tolerate the harder water, a lot of what's kept in your LFS is being kept in local water, it's well worth asking them. I'm from the South West and we have very hard water and I've kept several of the hardier, particularly farm bred, species with no trouble. You'll struggle with sensitive species, particularly if breeding them.
The options are essentially to find fish that suit what you have, find fish that are adaptable enough, or to mess with your water. I'm an advocate of not doing the latter unless you're planning to commit to a long term approach of doing it, remembering that things like large water changes need be possible in emergencies. RO isn't the only option, there are DI filters and similar that aren't so wasteful, and if you're in a good area with little air pollution then rainwater can be an option as well.
