I will say that an angelfish should not ideally be kept in a 20 gallon tank with other fish, as it can cause stunting, wich is likely what is going on here. The twitching could be from muscle spasams caused by bad water paramaters or stress. What are your water paramaters? They should technically be kept in 30+ gallons, unless you are an extremely experienced angelfish keeper. On a side note, your tank will be overstocked once all your fish grow up. Common pecos will grow up to 2 feet long, and require a minimum of 175 gallons per fish. Even smaller species like the bristlenose pleco need 30 aquariums. You say it is small right now, but what will you do with it once it is bigger? Sell it? Nobody will wants a likely sickly, stunted 6-24 inch pleco. Getting a fish you know will outgrow your tank in no time is very irresponsible. Albino sharks can grow to half a foot long are very active, and are aggressive, obviously not suitable for a 20 gallon tank with long tailed fish like angels and bettas. You will need to rehome the pleco and shark as soon as you can, and likely the angel as well if you truly want the best for your fish.
Disclaimer: None of this is meant to be rude or offensive. This is only for information's sake and the wellbeing of aquarium fish.