Temperatures are very important from speed of metabolism to health of fish. If the temperature is to low you can run the risk of diseases like ick, to high and you can starve or even cook your fish.
As for pellet feeding you can start when the fish is healthy and has a bit of weight on it, although you can try introduction of pellets and see if it is interested in them right away but remove uneaten food after 3 min to avoid contamination of your water.
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