My oldest lived to be 10 years old. Got him back in the 90s. Mom killed him. Ugh.
I had 12 at one point, most of them at 3 years+. Just got the three atm, but once I'm settled into the new place, I want to get some more (and also some shrimp).
I would just go ahead and put the betta by itself, but I'm overly cautious. They could coexist together, since the tank is so big, but again, I'm a wuss when it comes to putting bettas with most other fish. Shit, I'm still hesitant to do a sorority with female bettas.