He'd be fine alone[there's no fish that really HAS to have groups of their own kinds. They'd be happier, sure -- but it's not an issue otherwise as long as the fish is healthy] ... I'd be worried about him going after the betta's fins tho[mine were rather nippy & playful when I had them] and definitely passing a disease or parasite to him. Keep him in QT for a month and then if there's nothing you should be good.
2 dogs, bearded dragon in 40gB, 4 cats[ugh! xD], 2 turtles[RES/Painted sharing a tank with an oscar & jewel cichlid], 1 temporary 10gal "QT" tank, 55gal planted FW community & 15gal nano-reef ... plus a 20gL planted divided betta tank with 4 bettas that will eventually be rehomed and the tank - decommissioned.