Hello Chris. I know you from the Chameleon Hobby. Just thought I would jump into your thread and Say Hello after seeing you join and post here.
There are many FRT keepers here. I've seen some very nice tanks. One especially nice one with a large school of 50 Tiger Barbs.
Unfortunately as far as many common fish that you might want to keep with FRT, near 90F is higher than most would handle. If you lowered it slightly your options would vastly increase.
Also, be aware. This site isn't called MFK for noting. You are likely going to get some outrageous suggestions for fish to keep with your FRT. And consider that your tank is a mere 150 gallons, eliminating Gar and Arowanas.
Look into Tiger Barbs (Also note that there are several captive bred variations of Tiger Barbs, so While you can keep just the one species of fish, you can have colour and pattern variation in the fish within the school). This schooling fish grows to almost 3" and will school to avoid your swimming FRT.
Rainbowfish would be another good choice, many species are native to Papua New Guinea, and would share similar habitats as FRTs. They are extremely active, constantly moving, and aggressive feeders. They are easily satisfied on flake or small pellet food, fed several times daily. An automatic Feeder will keep them well fed throughout the day, so that they are less inclined to bother the FRT when you feed him. Most larger species tend to swim from in the middle upper region of the tank. You can mix species together. Be sure to have a large group of each though. I would put a very minimum of 12 in. Though triple that would do well.
All from Papua New Guinea:
http://www.aquahobby.com/gallery/e_boese.php
http://www.aquahobby.com/gallery/e_Melanotaenia_lacustris.php
http://www.aquahobby.com/gallery/e_praecox.php
http://www.aquahobby.com/gallery/e_Melanotaenia_parkinsoni.php
Also you should know that
if the Rainbows breed, they can Hybridise, which is frowned upon. Cull the fry unless you can be certain they are of pure species.
What does your salinity measure? (Be sure to buy a Hydrometer that measures well into the low end of the scale). The rainbowfish will tolerate some salt and if slowly raised could adapt to more significant amounts, but if you have it too high, you're limiting yourself to brackish fish (in which case, a school of Monos (fingerfish) would be really cool). Have you tried keeping it with very little salt? For fungal and Bacteria infections, Pimafix and Melafix are great for treating or preventing. They are natural tea tree oils and work well on FRTs.
And to complete your awnser regarding salt, I will say that is not unherd of them entering full sea water estuarine areas. Also I would go with sea salt, as brakish water has much more minerals then NaCl, its more complete and of better quality. Also arent dats potencialy agressive?
He is putting marine salt in, and posted sources that explain FRT's go in and out of brackish water in the wild.