Hi Scollin,
I have to agree with Pearl. A 1/2 gallon tank is tiny for a betta, and adding a second fish is not a good idea.
Plus, otos are shoaling fish--this means that they are social, and they need the company of their own kind. When kept alone, they tend to be stressed, which can make them prone to illness. It is often recommended that, with shoaling fish, you should keep a minimum of 6, for a healthy group.
Also, otos are notoriously delicate and difficult to keep alive. Many cannot be trained to eat anything other than algae. They may not accept veggies or wafers. If you do not have a reliable supply of algae in your tank, otos are not a good idea. I'm sorry that the person at the pet store gave you bad advice--but many people here, including myself, can speak from experience about the difficulty of keeping otos.
Here are some good articles:
http://hubpages.com/hub/Otocinclus
http://wetwebmedia.com/FWSubWebIndex/otocinclusart.htm
Also, note that bettas and otos have different temperature requirements: "
Most people keep their tanks far too warm for Otocinclus, which come from cool, fast-flowing streams and want something in the 20-25 degrees C range rather than the usual 24-28 degrees C most people maintain standard community tropicals at." --http://wetwebmedia.com/FWSubWebIndex/otofaqs.htm
Now, many people keep their otos in warm tanks, but 20-25 C is 68-77 F, and bettas prefer their water to be at least 80 F. Although, many people do keep otos with bettas successfully. The problem in your situation is really that the 1/2 gallon tank is just too small to keep otos in. Like Pearl said, you should return the oto to the store, and if you want to keep the betta with other fish, you should get a bigger tank. A 5 gallon tank would be fine for the betta alone. In a 10 or 20 gallon tank, you could have a betta and a small group of 6 or so cory catfish. Dwarf cories or panda cories would be good. Cories are totally cute, and are much easier to keep alive than otos.
Again, I'm sorry that the store employee put you in this difficult situation. But the internet is full of info. You can always research fish online before buying them, and that will save you a lot of grief (and a lot of return trips, believe me, I learned the hard way

)