NO! I have heard tons of reports of Serpaes being fin nippers, and so they obviously wouldn't work with a long-finned Betta. Actually I wouldn't recommend keeping any tetras with them as they may not be able to resist nipping.
a betta could potentially get along with smaller schooling tetras like rummy noses. but seeing as your already have serpaes and you are thinking of getting a betta I wouldnt reccomend putting it with the serpaes