salamanders are a negative...all the normally available species get big enough to eat or at least attack and kill a betta. also, some (like fire-belly newts) produce toxins in their skin which could also be dangerous. on top of that, most species need either a dry-land portion of the tank, or very shallow water, too shallow for for fish to swim in.
african dwarf frogs are a possibility. they tend to get mixed reviews on here as far as their compatibility, but from my end, i've always found them to coexist happily with my bettas and other fish.
there are several species of shrimp that can live with bettas, but be warned that your betta will probably end up eating some of them. crayfish are a BAD idea...bettas are slow swimmers, and a crayfish WILL catch, kill and eat your betta.
i've never had any experience personally with keeping other fish species in a sorority, but it's my understanding that it varies widely depending on the personalities of all the fish involved. if you go the sorority route, i'd have them up and running for a month or so before you try to introduce another species. that way you can gauge their group dynamic and have a good idea whether they would be hostile towards another fish in their domain. maybe buy the new fish from a store with an ironclad returns policy, and float them for an extended period so you can observe your girls' reactions, and if any of them begin flaring constantly, dart or charge the bag, or otherwise seem to be aggressive towards the newcomers, bring them back and get a refund. my sorority is species only, but i have a female crowntail in my livebearer community tank, and she gets along with everyone just fine.
So remember, when you're feeling very small and insecure,
How amazingly unlikely is your birth,
And pray that there's intelligent life somewhere up in space,
'Cause there's bugger-all down here on Earth.
Loki/Grendel/Gail/Beowulf/Camilla/River/Bathory/Arwen/Lafayette/Deb/Jack Skellington (soon)