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I have the same tank, often used as a quarantine tank. You might consider a couple of otocinclus, which would keep the algae under control. And maybe another cory cat to keep the first company. Your male Betta will provide plenty of color; the others tend to be dull. But I'm in favor of 'cleanup fish' that end up having their own charms. E.g., you can spend a lot of meditative time just trying to find the otos!
Also, not to dissuade you from a male Betta, but have you considered a female Betta or two? They're just as colorful albeit with shorter fins, and are peaceful (I have two or more together in other tanks). You could have 2 of different colors to provide more interest.