I didn't know if I could get a link to work, but this is from Mo's (A member here) Fish compatibility list;
The link;
https://www.bettafish.com/102-betta-fish-compatibility/97337-betta-splendens-compatibility.html
Ember tetras--------10 gallons----------six
Habrosus cories--------10 gallons----------six
hastatus cories------10 gallons-------------six
Von-rio tetras-------------15 gallons---------ideally 7 or more
Pristilla tetras------------10 gallons-----------six
Head and light tetras----10-------------------six
Cherry barbs----------15 gallons----------------six
Neon tetras-------------10 gallons------------ Six
Neons tetras can be a bit nippy when kept in small numbers so you would need atleast 6 to house them properly with a Betta
galaxy rasboras------------10 gallons-----------Six
panda and pygmy cories------10 gallons-----------Six
most other cories-----------15-20 gallons -----Six
harlequin rasbora -----------10 gallons---------Six
non fancy female guppies------10 gallons ------- Three
endlers livebearers------------10gallons---------Three
zebra danios-----------------3.5 foot tank-------Six
Bristle nose plecos------------2.5 foot tank-------One
Bushy nose plecos-------------2.5 foot tank------One
non colorful platies------------10 gallons---------Three
glo fish----------------------3.5 foot tank--------Six
rummy nose tetras------------15 gallons-----------Six
glo light tetras----------------10 gallons----------Six
cardinal tetras----------------15 gallons----------Six
X ray tetras-------------------10 gallons----------Six
"feeder guppies"---------------10 gallons-----------Three
Khuli loaches------------------2 foot tank--------Six
Scissor tail rasboras-------------10 gallons----------Six
Otos---------------------------10 gallons---------3 - 4
Bloodfin tetras-------------------15 gallons-------- Six
Rasbora Brigittae-----------------5 gallons----------Six
Black neon tetras------------------15 gallons-------six
Yoyo loaches
Shrimp that are compatible with bettas
Ghost shrimp---------------------2.5 gallons --------one
Amano shrimp-------------------2.5 gallons----------one
Cherry shrimp-------------------2.5 gallons---------one
and a little note about shrimp, they are a hit and miss, some bettas are fine with shrimp and some aren't but all of my bettas have been fine with shrimp that are over 1 inch. I would suggest that a well planted tank with lots of cover should be used when having small ghost shrimp or cherry shrimp
Snails that are compatible with bettas
Apple snails------------------10 gallons-----------one
Ramshorn snails------------5 gallons---------one
Malaysian trumpet snails---2.5 gallons--------one
Pond snails--------------------2.5 gallons---------one
Nerite snails------------------5 gallons----------one
Snails are a hit and miss too with bettas some will tolerate them but some will chop their feelers off
sometimes compatible fish with bettas
---Name of fish------------Minimum tank size------minimum group number
male, and female Fancy guppies----------10 gallons-----------three
Sparkling gouramis------------------------10 gallons----------one
bright and colorful platies----------------10 gallons-----------three
Black widow tetras----------------------20 gallons------------Eight to not be as nippy
sword tails----------------------------2 ft. Tank------------Three
Mollies-------------------------------25 gallons ----------3
Lyre tail guppies-----------------------10 gallons-------------Three
Fish that are rarely compatible with bettas
Including information regarding minimum group and tank size for this part of the list would most likely encourage people to further on and try the compatibilities which I wouldn't recommend.
Serpae tetras
other bettas
SA cichlids
NW cichlids
Dwarf gouramis
Tiger barbs
Honey gouramis
Angel fish
Honey gouramis
Boesemani rainbows
Red tailed sharks
Any type of crayfish with claws over half an inch