"Γεια σας, καλημέρα!"
Translation:Hello, good morning!
You would use Γεια σου to one person whom you know well. (Roughly: someone whom you would call by their first name.)
You would use Γεια σας to several people, or to one person whom you do not know well. (Roughly: someone whom you would call by their family name.)
Γεια alone is even more informal, so you would use it with one or more people whom you know well, but not to someone you speak politely/formally to such as a stranger.
Yes, say you're entering a shop in the morning, you might as well say both! It does not ring unnatural in any way. It is very common every Monday morning to wish "Καλημέρα και καλή βδομάδα!" and if that Monday is in the first couple of days of the month it even gets extended to "Καλημέρα και καλή βδομάδα και καλό μήνα!" as people realise they might have not wished a happy new month to their family/colleagues etc. It is a bit of an inside joke really at the same time, placing that much importance on pleasantries, because the phrase becomes sooo long and no one will take it personally if the extras are not added, but it is very common nonetheless.