Although I have never had to deal with a situation like that, I do have a few thoughts, some or all you may have already thought of, but here is my two cents:
1. Are you absolutely, positively sure that all the patient balances are legitimate? Meaning that the previous billers didn't move charges to patient responsibility that were supposed to be provider write-offs, or incorrectly billed claims that denied were then made patient responsibility, etc? Because if they were so lazy that they never worked the AR, and instead wasted time and money sending out 10-15 statements on some patients, then they may have been lazy enough to not do the rest of their job correctly. You wouldn't want to pursue a patient for a debt that isn't valid.
2. What does the financial policy of the office(hopefully there is one) that the patients sign say about balances and collections? That can be very important to this situation.
3. The figures I have read say that it costs about $10 to send out a patient statement, when you factor in labor, supplies, paper, etc. So if someone has a $60 balance, and has been sent 6 statements or more, any further pursuit of the debt will just lose the practice more money. I would not bother pursuing any balances that will waste more labor costs. I wouldn't even bother sending them to collections, as that takes labor cost to do also. I would just write the debt off, with a note in the account that the patient owes money, and to collect it before they are seen again. Of course, you can't refuse to see a patient if they owe you, but you can dismiss them from the practice for nonpayment.
4. I would work the biggest balances first, and during your phone call, try to establish why the patient hasn't paid. You can usually figure out pretty fast if the patient is a deadbeat who just doesn't care, or if they are truly having financial difficulties. Based on what you figure out, take it from there. People with financial difficulties can be offered a payment plan, with the understanding that one missed payment means the balance goes to collections. You could also offer a percentage off the bill to both groups, as an incentive to pay that day.
5. Whatever you say to a patient, you have to follow through in regards to sending them to collections. So you can't say you will send them, and then you don't.
And I will say Good Luck! Just reading your post made me itch. I hate, hate, hate when things like this happen to practices because people didn't do their jobs correctly. If my patient AR ever goes over $3000, I flip out, never mind 6 figures!