If the Pleco is small enough there's a good chance he could live for a few years in that tank. Mine is over ten years old and is only 11 inches long. Someday, eventually, I'll upgrade to a 75 gallon to accommodate for his size but just because he's a pleco, it doesn't mean he has to start out in a giant tank. Also, Plecos get along with bettas great and can even take a little intimidation and picking on the betta's part.
Also, there is no reason for him to have to take the betta back. I know it goes against the advice of lots of fishie forums out there, but you can cycle a tank with a fish in it. Plecos are some strong guys, at least mine is anyway, and bettas are fish that are victimized and mistreated by fish stores just as much if not more than they are beginning aquarium keepers. That fish will be fine in a vase or small bowl or even that aquarium until it cycles. Just make sure to get a test kit and do frequent water changes to keep the stress levels down. Also be sure to treat any water you do add because if you don't the chlorine in untreated water will kill off the good bacteria you're trying to cultivate.
Also, I'd hold off on the other fish. Cories are sensitive little things. They're super sweet fish but they need to be in big groups and I'd hate to imagine a betta picking on them. Same goes for the platy. I'm acting as a foster mama to two mollies that were being attacked by my brother's betta so I've seen how vicious they can be. If you want to attempt to add anything, (after your tank is cycled), I will recommend ghost shrimp. They're cheap, they don't add much to the load on your tank, but occasionally your betta might make a treat out of one of them.
If you know anyone that has an established aquarium you might want to see if you could get some of their filter media or maybe a porous decoration from their tank that bacteria might be growing on and introduce it to yours. It'll speed up the cycle on your tank and ease some of the stress on your fish. If you don't want to wait, you can buy something called Tim's One and Only. I've seen a lot of positive reviews on it and anything that can make the cycling easier on fish the better.