I'd add a load more neons to at least 10, and depending on the amount of 'floorspace' your tank has, you could add some cories. I don't think I've seen a 45 gallon hex myself before, so I'm unsure about how much 'floorspace' there is in them. Would you be able to calculate the area of it? If there's enough space, panda or pygmy cories might be fine in there. I wouldn't stock too heavily though as while it is rather large at 45 gallons, there is less contact with air than a rectangular 45 gallon and most fish prefer to swim horizontally as opposed to vertically. I'd guess that you wouldn't want more than 1 betta, 2 small schools of fish and some cories in there with some ottos if you wanted. Ottos like mature setups though, ideally with a fair amount of algae, so if your tank isn't similar to this right now I might wait a while.
I wouldn't recommend adding gouramis as they are of the same family and likely to fight. I'm unsure of bettas and platies.