It looks like you have a handle on the message types available in the Setup, Messages command, Timeslips can show messages to everyone ("Text For All Bills") or conditionally based on how old charges are ("Charges Due In Period 1" etc) or conditionally based on if there is an amount owed or amount due ("Debit Balance", "Credit Balance"). I don't think it can automatically show a message if it's the first bill for a client, or the second. But it shouldn't be too tough to do that.

An example:

- In Setup, Messages you could change one of the Global messages to "First Bill" and add text.
- In a template client, go to the Layout page of Client Information and set Message 1 to this new message, with duration of "1 bill".
- In Setup, General, New Names, use the template as a source of default information for new clients.

Now any new client will have this message set up for them and the message will stop after the first bill. You can use other message types mentioned above to handle the conditions of the 2nd+ bills.