Don't get any guppies. They resemble male Betta's, and there is a really good chance that the Betta will kill them.
Don't get a gold fish for two reasons. 1. They are a cold water fish, and Betta's are tropical, so they need a heater. 2. ONE goldfish needs at LEAST 20 gallons, with at least a 30 gallon filter, because they are super dirty.
Don't get any tetras or danios. They need schools, and this will overcrowd you tank and stress your betta because they are fast moving fish.
Don't get any Mollies, because they are actually brackish fish - They need some salt. They CAN be kept in full salt and full fresh, but they do best in brackish.
I would suggest 3-4 platties. There are lots of types, stay small, and don't stress your betta.
I wouldn't do any cory cats. They do stay small, bu are acually schooling fish, so you need 4-6. Pleco's get WAY too big.
Hope this helps out!