Bettas are generally fine with tetras like i said. My female is with six. Tetras stay out of the way (referring to the small types, like neons), they can easily swim away from the Betta if she is feeling a little grumpy that day... so they are fine companions.
The best thing to do is get the tetras first... leave them for a few days in the tank, then introduce the Betta in my opinion, then the tetras would've worked out all the hideouts (which you must have, perhaps some plants as well)... I can say they'll get along fine though.