The goby is also a filter feeder (Despite what the stores around here advertise, it'll only eat feeder fish if it's near starving), so it may be tough to feed if you're following store advice. I researched them a few months ago, and supposedly they can be acclimated to fresh water with some disease risk but shortened life (i.e. bad idea), and can be acclimated to fully marine conditions over a period of weeks. They're very adaptable to water conditions and have been known to be the sole survivors of tank-clearing disasters.
Here's the most complete info I found in my research. I shudder to think how much damage 75 ppm of nitrite or the ammonia required to produce it would do to any fish, even one able to survive in it.
To address your other questions, you've got a good group of barbs, so I don't think they'll bother most fish. You don't neccessarily need semi-aggressive fish (some semi-aggressive fish should be kept with peaceful tankmates, for that matter), but avoid anything with long fins which the barbs likely won't be able to resist, and likely stick to fast moving fish of a comparable size - the danios should be good, maybe black widow or serpae tetras.
The goby probably won't last in the long term unless you can find a brackish tank to rehome him. Since you have a plan to rehome the pleco, you can consider doing so sooner instead of later, and replacing him with a bristlenose, which is a very similar fish and more suitable to the tank size. I don't know much with rainbow sharks, but Alex is saying much like what I've heard in the past. Watch him for aggression, as they claim large territories, and don't add another shark (even a different species) to the tank, or they'll likely terrorize each other.
