The tank isn't even fully cycled yet so adding more fish at the juncture isn't the most viable soultion, aside from the fact that silver dollars get a little big for a 55 gallon tank. There are other tetras better suited for the job in this instance but now is not the time. I've owned JD's that were very cryptic and others that were more out in the open, if it comes out to eat I wouldn't worry too much. Just keep an eye on your water parameters and you should be fine. If the fish stops eating that's a different story but otherwise let it be for now.