Rainbowfish that have TB will produce the following symptoms before they die.
Stop eating, bloat up overnight, do a stringy white poop, sit up by the surface or at a filter outlet gasping, die within 24-48 hours (usually within 12-24 hours) of producing these symptoms.
If the fish loses weight over a period of time or gasps at the surface for a week, it isn't Fish TB.
If a fish keeps eating after it does a stringy white poop, it isn't TB.
The fish TB destroys one or more internal organs of rainbowfish, causing them to bleed internally and die from internal organ failure.
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Is there a risk to other fishes?
Yes. Any fish can catch it if they are kept in an aquarium with a contaminated fish. Fish can also catch it from nearby aquariums that are infected if you use the same equipment (nets, gravel cleaners, plants, filters, etc) in other tanks.
Rainbowfish are extremely prone to the disease because they haven't had any previous exposure to it, unlike older types of freshwater fishes that have evolved with the disease over millions of years. The other types of fishes do die from TB, but they can live with it for longer.
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Are rainbowfish worth keeping?
If you can get disease free stock, yes, they are wonderful fish to keep. But unless you can guarantee they are free of the disease, you could be planting a bomb in your tanks that can wipe out every fish you own. And the only way to get rid of it is to destroy all livestock and disinfect all tanks, gravel, equipment, and everything associated with the fish, including stands, nearby furniture and walls next to the tank.
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The only way I would keep rainbowfish (or any fish) again is if I had wild caught stock that had not been kept in pet shops, wholesalers/ importers, or in tanks that contained any captive bred fish or plant from a fish farm (doesn't matter where the farm is located).
If I keep rainbowfish again, I will go out and collect my own. I would add them to new tanks that have never had any other fish in. I would keep aquarium plants outside in full sun in summer in Western Australia and grow the plants out of water. I would then take cuttings from new growth and use those cuttings in the tanks. Or I would collect seeds from the new growth and plant the seeds in clean tanks.
Other people will just get fish from pet shops and accept the losses. Some might get fish from breeders and hope they don't get sick fish. You can try getting eggs sent over and moving the newly hatched fry into clean rearing tanks. It's a risk whichever way you do it. The fish you get might be free of TB or they might have it. Unfortunately there's no way to tell until they start dying, which can be a couple of years later. Then it's too late.
The only way to be almost 100% certain they are free of the disease is to go and collect your own from the wild. Even then there is a chance they could catch the disease from other types of fish they are housed with, (eg: cichlids, catfish, barbs, tetras, etc).
Plus the main focus of the tank I want to do is a Nicaraguan Cichlid so there is going to be a mix, the goal of the tank is to raise a Nicaraguan long term so if the Rainbows are in someways a risk maybe I need to reassess that... 
