Your tank is a bit overstocked. Normally the rule is 1inch of fish per gallon but not in all cases. If the gouramis start to bully or attack each other I would re-home one of them so they don’t get severely hurt each other. The school of neons and rummy noses are ok size wise but I would add 3 more Cory’s. If your son agrees it might be a good idea to change the substrate to sand so it is easier on the Cory’s whiskers but if not the gravel is good. Some more plants maybe even some live plants would be good to help the fish hide a bit such as Amazon swords, red root floaters, and hornwort. If you add the live plants I would add some liquid fertilizer as the directions say.