There are some fish that I have good luck with in with the bettas but danios don't sound like they would be compatible. They are members of the same family as barbs and rasboras and they inhabit the same space as the betta, top level. They are also very active and the bettas do tend to be a bit skittish about really active fish. They need quiet, calm, peaceful tankmates that inhabit the bottom or mid levels of the tank. I would recommend something like Otocinclus catfish who are mostly nocturnal and when they are out during the day don't get in the bettas way. If your tank is big enough you could put a group of cories in with the betta but they need a lot more room.
It ALWAYS depends on the temperment of the betta, there are some bettas who will accept any other fish in with them even the ones who are not good for it, and then there are bettas who will harass or kill any other fish put in the tank with them. It is always a good idea to have whatever fish you want in the tank and THEN add the betta, BUT be prepared with another tank to put the betta into if there are conflicts. That is really the only safe way.