1.020sg is already full-strength seawater, if you go for this salinity, then your filters will not be brackish anymore but will adapt to seawater-strength. I guess is that people who wrote 1.015 to 1.020 sg are assuming that you want to eventually keep the eel in seawater. Pure brackish is 1.010sg, so if you want high brackish then the range should be between 1.010 to 1.015 sg.
I will get back to you when I have more info about compatible fishes.
Actually, A friend just told me that dragon gobies could be kept successfully with various types of brackish and marine morays. However, as you might have guessed, the dragon goby is a brackish fish that is rare, and probably rather expensive. Plus it also has certain requirements of its own, so I can't say it's recommended.
I also heard that ropefishes (erpetoichtyes, a relative to bichirs) could tolerate low-end brackish water & won't bother your eels nor hogged all their food, but since you want to keep your eels in high-end brackish, maybe it's not so good idea to get a ropefish.
I'll search around to find another possibilities, so don't lose hope yet