Corys are amazing little fish, stay relatively small and mine do a pretty good job at cleaning up excess food. Things to remember with them though:
They need to be in schools of at least 4+ to be happy so a small tank wouldnt be able to house them. 10g with a good footprint is ideal.
The cannot live off just scavenging and require a ballanced diet much like your Betta.
Substrates are limited due to their sensitive barbels (whiskers). Sharp or large gravel will damage them so sand is best and more rewarding.