Well, you can get like a 5 galllon tank because my pleco loves it and he's always happy. Does he regularly swim to the top of the water then goes back down? (as if hes jumping for air) If your betta builds bubble nests then the pleco will ruin it.
Regardless of the species of pleco, a 5 gallon, let alone a 1 gallon, is wayyyy too small. Bristlenose are the smallest, only getting 3-5 inches. a 10 gallon is needed for one and a 20 is needed for 2. A common pleco needs at least a 55 gallon. Too small of tanks can cause stunting or stress. And stressed fish are much more susceptible to illness and premature death.
I would find someone to take him. If you know anyone, or try craigslist. Or bring him back to the store. And wal-mart is just about the worst place to buy fish. They know absolutely nothing, will never give you right information, and, as a whole, take the worst care of their fish. See if you have a local or chain pet store around for buying fish. And always do research before bringing an animal home. Our sister site tropicalfishkeeping.com has fish profiles with good, concise information on tons of species, it's a great place to start.
Please don't just stick him in a 5 gallon. It's too small. Either take him back or find him a proper home.
If you cant find anyone to take the pleco and price of a larger tank is the issue, you can use a large, clear storage container. They come in all shapes and sizes. You can probably get a 10 gallon one for like $10 or so. I know they have even larger ones as well. I am having "issues" with my 20G and a mystery illness outbreak and 5 of my females are now in a storage bin.
i had a pleco in a 29 gallon. he/she was nearly a foot long, and lived about 11 years. :T the only reason he, and the goldfish he lived with, died, was because my dad would put random baby fish he caught while fishing into their tank, and they caught sometime. so... they need HUGE tanks. bigger even than my 29 gallon.
I am so happy to hear you are able to find him/her another home. Our pet store sells plecos as algae eaters and great for any size tank, and if they out grow your tank you can bring them back and trade for a small one again. They then sell the big pleco for $20. They make $30 off each pleco. Anyways, plecos are a no no. So happy you listened. They are very interesting fish to have though, I have one (Ms Pleco AKA Jaws) in my 29g. She's 6" right now, and I bought her a 50g tank to move into next spring :) She was dumped on my doorstep when I bought a 10g tank :*(