Platies and tetras look very nice together. If possible try to put the smaller tetras first and then the platies. This is to cut aggression (if any). However, red tail sharks are a no-no as it's very aggressive and temperamental. Could just take a nip off your platies!

Water change should be done on a regular basis; maybe 20% every week. This is to ensure that your fish doesn't suffer the 'shock' from total water change and also chemistry fluctuations. If your filtration system isn't mature yet ie. there isn't enough nitrifying bacteria to neutralise your high nitrites; you might cause death of all yr fish. Therefore it's important to fully cycle your tank before adding fish. Make sure you have test kits on ph, nitrites and ammonia to do regular tests, until readings are 0.

What filtration system are you planning on having?