I'd say it should be alright once the tank is growing and established. A lot of users feel that danios are a little zippy and quick for a betta to feel comfortable. Since you are using a 20g long, tetras should be ok since there will be plenty of room for the proper school (at least 6, preferably 10) to do their own thing and the betta to do his or her own thing. I'd make sure the betta has a place to get away from the school of fish if he or she so chooses.
I'm not sure about the platys. Try doing a search of the forum and see what past users have said.
With bettas it is always a trial and error type thing. Some bettas do fine with other fish, some go crazy. Just make sure to have a backup plan.