Wow! A lot of answers from folks who know little about Bettas.
If your aquarium is large enough, by all means. The danios will be fast enough to get away if it did decide to attack them, which is highly unlikely. Daniios are schooling fish, so would need to be kept in groups of 6 at the least. They are also very active swimmers and prefer a long tank rather than a high or deep tank. This would also be best for the Betta and the catfish as well.
Corydoras would be an excellent choice, but they also should be kept in schools of at least 6. There are many, many species of these, and a lot of them are very attractive.
If you only one one catfish, consider a pitbull or clown pleco. They will keep your aquarium algae free, and will probably need to be supplemented with wafers and green vegies like sliced zucchini or sliced green beans. Canned green beans are great and readily eaten, but need to be well rinsed to remove the salt.
Male Bettas are more colorful, have long flowing dorsal, anal and caudal fins. Females are usually rather drab with short fins. At least with Betta splendens, the one will usually see in pet stores. In this species, the male will also build a bubble nest at the surface, the female will become very rounded as she fills with eggs.
Bettas usually only attack other fish with similar finnage and bright colors. Paradise fish would be included in that, and possibly some fancy male guppies.  | 
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