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Oh no! A common pleco can reach up to 2 feet so needs a giant tank... The 20gallon is not big enough for just that one fish. Can you return it to the store?
You'll need to get rid of the pleco soon as it will grow too big. Otos are similar but are much smaller and more appropriate for a 20g.
If you want to keep it for now, I wouldn't recommend buying anymore fish as your stocking is quite high already. Maybe add 3 more of the barbs so they have a school of 5?