Would these tankmates be compatible with my betta fish also known as the siamese fighting fish?

Barbs are just too aggressive to share the tank with a beta. They'll pick his fins ragged. I've seen them at the fish store in the same tank with guppies and mollies. As to the size of the tank, I've been assured over and over that betas prefer a smaller tank because they can get to the surface more easily and then "hide". The biggest difference in activity for my fish is the temperature of the water, and the smaller tanks do cool more quickly. Make sure your water is warm enough for your beta, and he'll be a happy, active boy.