Long-finned Betta splendens are bio-wrecks. They have a hard time swimming, and if tankmates start nipping and ripping, they can't defend themselves. Unless your tank is tiny or overstocked, they won't harm tankmates (based on the many I kept over the years)., They can't catch them.
If you don't think and put them with fancy guppies, okay, carnage. But to put a banner finned fish with a banner finned fish bred for fighting is not wisdom from the get go.
I always kept them in communities, but I never bought large finned individuals. I would look for more compact fins - the old blue and red solid colour strains, if I wanted them for a community.
If you like the big fin mutants, then a solo tank is in order. Human intervention and manipulation has crippled them.
Wild betta splendens can handle sensibly chosen tankmates with no problem, but they were not selected for aggression or for finnage. But finding them is very hard. Pla-Kats can do damage, as they have been bred to fight, and can move.