It'll be okay if there isn't any other seed mixed available at the moment though they shouldn't eat only a Budgie seed mix. Finch and Budgie seed mixed are ALMOST the same but there are a few differences.

Yes it is normal to feed a zebra finch parakeet food. They however can not eat larger bird food such as cockatiels, lovebirds, etc. Even if you gave a zebra finch cockatiel food the finch would pick out the food that it could eat.

Generally zebra finch food and parakeet food is the same.

They will pick out the food that they want, so yes it i fine for them.