Wow...apologies for the late reply.
Inverts do not handle lower salinity well at all.
That is not recommended at all. Think about it, parasites are invertebrates. We are doing this low salinity treatment in order to kill parasites, it will likely be very hazardous if not fatal to other invertebrates.
IME, fish dont have a problem with lower salinity. However, there's a couple of things to keep in mind:
Fish can tolerate big drops in salinity, however, increases in salinity should be done slowly. I havent had a fish have a problem with dropping salinity from 1.026 down to 1.008 in one setting. But you need to WATCH YOUR pH!!! Low salinity water has a harder time maintaining pH. Doing a slow drip of RO water with either Soda Ash or Kalk to maintain pH isnt a bad idea. The RO drip maintains your lower salinity, and the Kalk/Soda Ash maintains your pH. You also really really really need to be careful of ammonia in a low salinity environment, (1.008). The bacteria that processes fish waste/ammonia doesnt die off in low salinity, it basically goes dormant and doesnt process the ammonia very well, (if at all). Ammonia build up can be lethal to fish, very quickly. Regular water changes and ammonia monitoring are important.
30 days of low salinity (1.008) will kill off ich and should seriously drop the fluke population in the fish. I would also expect that any sort of parasitic isopods would have a tough time surviving 30 days of low salinity (1.008). However, from what I've read, internal parasites such as worms can handle that low salinity for longer than is good for the fish. To eradicate worms, (especially in fish such as tangs and rabbitfish), you will likely need something like Prazipro.
As far as keeping a QT tank at 1.008 regularly....I dont think that makes alot of sense.
It's going to be time consuming to maintain that level of salinity, and when there are no fish present, it doesnt make much sense to spend the time and effort IMO. For fish, just keep mixed salt water on hand, drain and clean the QT, and put away until next time.
For QT'ing inverts you're probably going to want a dedicated tank without fish that will remain running continuously.