Catfish?, get a red tailed catfish. That would be perfect for your nano aquarium...........nah, I just joke! Don't get an Asian bumblebee cat and put it with shrimp. If the shrimp can fit in it's mouth, he will eat it. Also, if I recall right, I believe that the Asian bumblebee cats grow to about 5 inches, at that size he'd be a threat to your beta. Go get some pigmy cories and call it a day.
He's right. Your aquarium is not big enough for neither fish. I was just playing around. Either way, enjoy your betta. It may lead to bigger better things. Keep it simple and have fun. When your hobby starts to feel like your job, you may lose interest in it.