The small amounts of IO you would be using would have little if any effect on pH. Furthermore, pH would likely increase whether you used large quantities of IO or FW
salt. I used IO for years in reefkeeping. I had problems keeping my pH high enough. It had nothing to do with the salt, it had to do with my house being so "tight". I could open a window and the pH would increase pretty quickly.
Replicating nature is usually best. For example, folks used to advocate keeping lower than natural salinity in SW
tanks. They claimed you kept more oxygen, had less problems with ich, ... . The reality is that water with salinity lower than natural natural seawater is often detrimental to invertebrates. Fish can stress too. There is really no reason to keep a SW
tank @ a lower salinity than what occurs in nature.
Why not go with what is natural? If brackish water fish live in areas that are brackish due to their proximity to the ocean, why not try to replicate? How can anything different really be better for the fish? JMO