Is this big enough for my betta and another fish?

NO WAY!! that looks like a drinking gl to me and theres no way even the betta will be happy alone in their theirs just no space for it to swim and grow, also putting anything besides a female with your male betta will result in the other fish being picked on and most likely being killed. Bettas are very teritorial and wont hesitate in such a small space to kill the other fish, i've had a betta kill plecos twice its size before, they can be mean little guys.