Is it it normal that penguin tetras attack zebra tetras?

I have two of each. One penguin tetra stays at the surface while the other one stays most of the time at the bottom. When they see the zebra tetras near by the penguin tetras chase them.

Is it normal that zebra tetras eat newborn guppies?

You might have squabbles between tetras, although these are usually not serious. I am not sure which fish is the zebra tetra though. Are you referring to the zebra danio by any chance?

It is normal for fish to eat other fish that they can swallow in one gulp. So, if the newborn guppies fit into the mouths of another fish, including tetras and danios, they will be eaten.

Joseph

Yes zebra danios.

What is the least number of tetras in group could be best kept (like neons and penguin)

It is best to go for a group of at least six, although more would look better and the fish will probably be more at ease.

Of course, you should first consider your aquarium's maximum capacity.

Joseph