Zebra Danios are nippy, period. It has nothing to do with compatibility, tank size, breeding or stocking. Their nature is that they are active little fish who tend to chase and nip. There's really no need to argue this point. Rarely do I step out of my character and say something is written in stone, but on this point I will do so. Zebra Danios are nippy fish by nature. Period. I've been in this game for almost 30 years...trust me when I say something is solid, because I'm the guy who likes to argue against those who make blanket statements.
Now whether or not they are dangerous or deadly depends upon other factors. Those are what must be considered whenever you put them in with other fish. Most danio chasing/nipping is relatively harmless, but that does not mean for a minute that I would place them in a tank with a male betta. I would also exercise caution in having them with any other fish that are peaceful, long-finned, or slow moving. It has nothing to do with the size of the aquarium or the size of the school of danios, and everything to do with their nature of being nippy. I'm sure that some are more docile than others, but that is a factor of chance and often not worth finding out over the remains of your prized betta's tail.
As far as lemon tetras...I think they'll work out okay. This is because lemons are also fast moving, active little schooling fish so the danio chasing shouldn't cause them any serious harm aside from stress. But some stress is good; it keeps the fish moving and in shape. After all, conditions in the wild are hardly without some of these games and dangers.
Just keep an eye out for any serious issues, and other than that, let it be.