This is the specification, in fact 9 V suffices with most mice. The mouse driver can recognize a mouse by dropping RTS to low and raising it again.
Various mice can operate at different speeds.
The first byte of a data packet can be used to synchronisation purposes. Data is sent to RxD in 3-byte packets. However, there are some three button mice which also use the Microsoft protocol. In particular, a press of the middle button is reported as 0,0,0,0x20 when no other buttons are down. Data is sent to RxD in 5-byte packets.