Yea, it's weird how that works. If you have 1 employee, he sets the stage for all the following new hires... I would sit the employee down or take him out to lunch and just be honest.
Something like 'Look, when you first started here, we were a small shop that wasn't that busy. Since that time, we have grown to a larger business and our customers are starting to expect more from us and so I have to expect more from my employees.' Let him know you would love to keep him around and let him grow with your business, but he needs to realize that the new employees look up to him. If he can't follow the rules, disciplinary action will follow. I wouldn't be rude or strict about it, almost play it off as if your hands are tied (really, they are tied. Your customers expect a certain level of service from you, your employees must realize that.)
That's how I handled it at my shop and it has worked so far. It's not fun letting employees go. Even if you are afraid of being painted as an 'evil and greedy boss', you can always explain that it costs you more to fire someone, find a replacement, train the new hire, additional taxes with new employee, etc. It really is in your best interest to keep the employees you have (seriously, ask whoever does your taxes).
My 2 cents, good luck. Sounds like you're having growing pains which is a good thing.