You should use this tool to make sure that your tank will not be overstocked with what you plan to add to it:
http://aqadvisor.com/
Keep in mind that most fish that are compatible with bettas are social, schooling species that need groups of at least six of individuals to feel secure and express their natural behaviors. Danios and tetras can become aggressive and severely nip at your betta's fins if they are kept in low numbers. Pygmy cories are one of the best choices as far as betta compatibility is concerned, their size makes it easy to fill out a decent sized school and they are unlikely to illicit aggression from the betta due to their bottom-feeding nature.