What is the command to fix the serial rate? Can I add in a string with other commands, such as S0?

Fixed DTE Rate +IPR

Command syntax AT+IPR=<value>

Description: This commands specifies the data rate at which the DCE will accept commands.

Values:<value>Baud rates that can be used by the DCE

0 Enables autobauding
300
600
1200
2400
4800
9600
19200
38400
57600
115200

If you want to concatenate multiple commands together on the same line, then use a semicolon to delineate the commands.

i.e. ATS0=1;+IPR=9600