Fish food is made largely of crushed fish, crushed meat and meat derivatives, dried out thoroughly. It lasts about a year or two provided it is kept VERY dry and in airtight containers. If it's still sealed, chances are it will be fine. If you open it and it is crusty, smells bad (offensive or rotten as opposed to smelling strongly of fish) or is damp, throw it away.
Light damages fish food, so store it out of direct light and keep it cool. I store my fish food in the freezer and I use small jam jars to hold a small amount. When I use that, I refill the jars from the main containers in the freezer.
As for whether coldwater food is suitable for tropicals, this will depend on the nutrition analysis and is rather complicated. Goldfish are herbivores and goldfish food is suitable for tropical fish if not fed as the sole diet - alternate with a feed intended for tropicals, preferably one containing a large percentage of meat, shrimp, squid or fish to balance out the wheat or soy base of the goldfish flake. Fish food will keep for years in the freezer so buying more is no disaster. Generic coldwater food may not be suitable as it often contains a very high proportion of lipid (fat). If you do feed it to your tropicals, feed it as a treat no more than once a week as a large proportion of fat in the diet of tropicals can lead to death from liver damage.
Don't feed pellets unless they will fit in the mouths of whatever fish you are feeding them to. If the fish can't eat them, they will ignore them, and they will rot and send your ammonia through the roof. Spirulina is extremely good quality food, but a lot of foods sold as 'spirulina' contain very low percentages of spirulina algae - it's used as a marketing ploy because people know it's good for their fish. At any rate, it's probably suitable for most tropical fish.
Most fish food containers will have an analysis on the side giving percentages (often on a min and max basis) for protein, lipid, moisture, ash (indigestible materials) crude fibre etc. If you post the percentage analysis I'll be able to give you a far more accurate analysis.
